Planning 7 - 12 moves in advance?

This is something I don't get and I highly doubt GM's actually do this. Let's say you're thinking about your moves and you say "If I move here, he will do this, I will do this ect ect" and then you move that peice, he moves the peice you thought he would, then you move your peice, then he moves a peice you didn't think he would and now your entire chain of moves is messed up.

So let's say your chain of moves you calculate is a,b,g,h,d,t,k,o,l,s which is 10 moves. You might play a, expecting him to play b but he plays t instead. So taking that into consideration, now you need to calculate:

a,g,b,h,t,d,k,o,l,s

a,b,t,g,d,t,k,o,l,s

ect ect which is almost impossible... with only a few minutes to make your decision. When I watch GM's play matches, they only seem to calculate 2 moves at a time, usually trying to work towards a goal of some sort but they really do seem to make it up as they go along.