I think Ruatha has guessed the right answer. If you had started the download and made some progress, then cancelled or been disconnected for some reason, it would count the already existing portion of the download in the speed calculation when you resumed.
I'm on dial up too and for the longest time I could never get more than 28.8 . Don't know where you are (no need to tell me) but since I've moved closer to a major street I now connect at 36k or 37.7k . I also agree with the theory that more people are off dial up now and that frees it up for those of us on steam powered computers and Beer Can / String connections!
I suggest you try to duplicate the conditions and see what happens.
I was sooooo tired when I made posted this Last night.
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Creator of the Star Trek Mod - AST Mod - 78 Ship Sets - Conquest Mod - Atrocities Star Wars Mod - Galaxy Reborn Mod - and Subterfuge Mod.