As for why, no way to know for sure without inside info, but best guess is that they are trying to account for maximum file size limits across all the various possible Windows/Mac filesystem types but whichever employee setup the Linux ones realized that most Linux users wouldn’t be using shitty Microsoft filesystems. FAT12 is fairly safe to ignore but they might have been considering FAT16 and HFS as the lowest common denominators, then making the files slightly smaller than the max file size just in case.
That or possible that they were balancing by network loads (since Windows versions probably account for around 99% of all downloads) and that was somehow determined to be the sweet spot.