I don't think there really is any reason for it. I think that it is just because people like round numbers, not decimals. Also, if the increase is really small, like only 0.25 Mb, then it really isn't worth going through the pain of hard forking for such a small increase.
So there's no real reason except marketing. Also, I don't think that they should do an hard fork, whichever number they choose.
Because they're not based on science.
How could they be based on science ? Please explain.