Opening popularity stats

If I'm white, I play 1.e4, and black has responded 1... e5 in the last dozen games. Then, I go 2.Nc3 Nc6 3.Bb5 and a Ruy Lopez (of which I know nothing, I think I should study it a bit) happens. I almost never see 1... e6, ...c6, ...Nf6, ...c5, and so on.

If I'm black, and I see 1.e4, I go 1... e6. More often than not, I see either 2.e5 or 2.Nf3, I don't see 2.d4 very often. Again, not the main way of playing. If white goes 1.d4, I go 1... d5, and I rarely see 2.c4, I see a whole lot of other things, something like 2.Bf4.

I'm not saying that these are bad, I am a bad player so it makes no difference if I don't see main lines. But, it interests me when I see players, who are fine at knowing where pieces belong (I wish I did) choosing odd moves on move 2 or 3, and the following question interests me:

What difference is apparent (in percentage terms) regarding move choices in the opening between different rating bands, and why?

Can it be that everyone is avoiding main lines in lower rankings? Why? Or is it that people don't know any opening theory beyond move 2? Again, I'm not saying that people should know any opening theory beyond move 2 at this grade. However, I would assume (because it is true for myself) that curiosity would get the better of everyone eventually, and they might think, "oh, if he/she goes ...e6, what is the "normal" response"?

Well, there are many reasons for this.

First and foremost, like yourself, most beginners have no clue what the openings moves are, as is proven by your "Then I go 2.Nc3 Nc6 3.Bb5 and a Ruy Lopez".  Uhm, 1.e4 e5 2.Nc3 Nc6 3.Bb5?! is NOT a Ruy Lopez.  It's a bad third move in a Vienna Opening.  The Ruy Lopez is 1.e4 e5 2.Nf3 (NOT 2.Nc3) Nc6 3.Bb5.

Secondly, when you talk lines like 1.e4 e6 2.e5, it's because not only does that beginner not know openings, but also has no clue of opening concepts either!  So instead of a "learning beginner", here we are talking about "clueless beginners" who "apparently don't even care!"  In the case of 1.e4 e6 2.Nf3, half of them will play that because they are following "Opening Concepts" about developing pieces toward the center and not making too many pawn moves, but still don't understand that 2.d4, taking even more control of the center, is better.  The other half of 2.Nf3 players may actually know what they are doing, and playing what is referred to as the "French Wing Gambit" if they follow up with 2...d5 3.e5 c5 4.b4!?

The reason for a high rate of 1...e5 is that most of those that don't know openings do at least know the concept of taking their share of the center, and so that is what they do.  Obviously at the higher levels, 1...c5 is the most popular.  However, 1...e5, 1...e6, 1...c6, and to a lesser extent, 1...d6, 1...g6 and 1...Nf6 are all common moves.

As for 1.d4 d5, yes, 2.c4 is the "main line", but it's not White's only option.  2.Bf4 may be played first by some London players, and 2.Nc3 by Veresov Players, but the most common second move if you exclude 2.c4 would be 2.Nf3 by far.  It leaves the options open for White.  After 2...Nf6, White can play:  3.c4 (Transposing to a Queen's Gambit), 3.e3 (Colle), 3.Bf4 (London), or 3.Bg5 (Torre, though it's not good here, Torre is better used against 1...Nf6 and 2...g6 or 1...Nf6 and 2...e6, not 1...Nf6 and 2...d5 [or vice versa] because of 3...Ne4).

However, until you reach 1800 over the board (ratings here don't count - Blitz ratings are meaningless, correspondence chess is a totally different game), forget about openings, forget about opening statistics, forget about all of that.  Learn opening concepts (control the center, don't move pieces multiple times unless forced to do so, etc), and do a heavy dose of studying on Endings, Tactics, and Positional Play, in that order.  Whether 1...e5 or 1...c5 is more popular at a given skill level should be about as important to you at this point as what shade of tooth whitening would look best on Oprah Winfrey if she needed a cavity filled in her top right front tooth!

In my case, it's probably because I don't know the exact move order or know what the precise main line is in many cases.

As a lower rated player, I would recommend getting Jeremy Silman's book The Amateur's Mind first. It does explain some basic chess principles, but it definitely demonstrates some psychological bad habits that lead to poor chess which we want to avoid.

I'm going to say something heretical here and suggest that learning an opening at your level (~1300 online turn) is worth doing. However, just pick one for now. I'd recommend something straight forward and solid like Queen's gambit as white.

As you have discovered most players at your level will respond to d4 with d5, so you will have plenty of opportunity to play c4. Many players will accept the gambit (immediately take the pawn) and try to hold the pawn, you can learn how to punish them for that* which will be fun.

Often of course players at your level don't follow book openings, so you will frequently need to fall back on the basic principles of openings as they go completely off book.

http://www.chess.com/article/view/the-principles-of-the-opening

Having a standard opening that you are playing (most of the time as white) will help establish some structure and make the near limitless options less overwhelming. You want to learn not just what to do, but get some idea what to do when your opponent does something wrong. This is a nice link for some info along those lines.

Exploiting Typical Opening Errors
http://www.chess.com/chessmentor/view_course?id=334

Or course as others have said, you want to analyse your games look for mistakes and get better at tactics. 

* There are some ways for black to hold the pawn in QGA if it is captured later, but that is a more advance level.