for one, the professional chess player has much more knowledge ( of actual chess positions and tournament experiences ) to draw from. A Grandmaster is familiar with literally tens of thousands of different type positions that will prove useful when competing.
Even if amateurs had access to those positions, the professional player gets to the heart of the position, knowing what elements to evaluate first much quicker than the amateur ( let alone the amateur may consider other elements of the position to be more critical than the pro ).
Of course there is a difference between a thinking of a professional chess player and a normal chess players. What is that thing which creates such a great difference?
PLEASE FREE TO ANSWER MY QUESTION.