The fan survey takes a small sample of fans , so it can fluctuate week to week. This has been said many times, and it is in keeping with the overall "keep things hazy" approach to how the game actually works. Your best bet is not to put too much stock in the fan survey. Instead, view it as a very general assessment of how full your arena will be in the next game. Also, I think the overall rating is the one that matters.
Once I scored a basket that still makes me laugh.