Maybe Photoshop only gives an estimate at what the filesize would be if you saved it in a specific format at a certain quality? I know Fireworks does this, and Fireworks can actually give you up to 4 estimates (maybe more but I havn't seen) for different file formats and qualities which you can choose yourself so you can see the effect of saving in different formats without actually saving. The file size that Explorer tells you though after you've saved is going to be closest to the size of file that everyone else will get once you've uploaded it though.