[IMO] Luke Thomas broke this match-up down the best way it could possibly be broke down.
It's pretty long so if you have the time, it's well worth the watch.
If not, basically he explains that as far as negotiating goes, GSP is in a tricky spot when it comes to fighting Anderson Silva. The closer to 185 this fight happens, the more advantageous it is to Silva. But if Silva were to make a successful cut to 170, it would only make sense for this fight to be for a title. So the best advantage for GSP without possibly losing his title is for the fight to happen at around 172 - 174 lbs. But at the same time, people would start questioning how much of a difference 2 - 4 extra lbs make when it comes to how the fight turns out.