.... Call it placebo if you like, but the ROM install did not work well (on my phone) without 2 wipes. And I tried 3 times because I know enough about how computers (should) work to laugh at the idea 2 wipes being better than 1 somehow.
There is enough (random seeming) variations in how our different phones handle these things that I don't think you can call it placebo just because you never had to wipe twice to get Froyo working on your phone. It makes no sense, I agree, but it was also my experience.
Experience, of course, isn't scientific testing. Its enough of a pattern for me (I tried 3 times to install Froyo with one wipe and the first time I wiped twice it worked. Twice.) that I shrugged and said "Whatever gets the d@mn thing working."