Game Explorer database

I'm curious... in the Game Explorer, I can select to explore "Master Games".  That's nice, but where is this database from?  Who are these players?  I never recognize any names, and when I compare against my personal database (which is games from 2700+ players, I think?), the results are often different.

Not that I'm complaining, just wondering about the strength of these players I'm looking at.

we compiled several huge databases of master games. admittedly some weird low quality games slipped in, but 99%+ are master games.

So "master games" means... what?  NM/CM/FMs?  It's not that I mind having that resource available -- after all, they're still much better players than I.  But I'm wondering about the omissions, particularly important games played by high-level GMs.

For instance, I'm in a Leningrad Dutch tournament that recently started.  One of my opponents played 7...e6, which I know also was played by Botvinnik against Tal in their 19th match game in the 1960 championship.  There are games in the Game Explorer that follow that line, but that's not one of them.

Doesn't it stand to reason that many (most?) theoretically-important lines come from games by top grandmasters?  What conclusions can I draw about lines deep into the Najdorf, for instance, without including any results by Kasparov?  Is it intentional that these are missing?  Are there copyright issues involved?

Again, I don't mean this to come off as a complaint.  I've just been using the Game Explorer a lot, and am trying to gain an understanding about what that database really represents, so that maybe I can reassess my approach to using it.

Hey, I'm bumping this up as my question is sort of along the same lines.

How often is the database here updated with master games, and how complete is that update-- with regard to the big tournaments and matches?

Also, is there a quick guide to what the sources of the game are?  I don't often know if the game was played under normal time constraints or some kind of rapid chess.  I'm guessing "It" means international tournament-- either that or Italy is like the chess capital of the world (which would be awesome if that was the case).  Could we some day sort games according to the time controls?

This just seems especially topical in light of Jeremy Silman's recent articles on studying master's games.