The reasoning behind them is:
Lucida Grande for those on Mac OS X (it's the system font and so is uber-tweaked for on-screen display using the Macs display layer technology)
Verdana for Windows users, especially those on pre-2000 systems without anti-aliasing as it, like Trebuchet, is designed expressly for that purpose, though I think more successfully and less showily. Ironically, that is the reason behind its 'fatness'. read more at MSFT
Bitstream Vera Sans for those on more modern Linux desktops who have not installed (or can't install) the MSFT fonts.
Geneva to catch any Mac OS 9 users, like Verdana this font is specifically designed for non-anti-aliased display (you can see it in your iPod if you've got one)
Lucida is a fairly common and somewhat screen legible font on older Unix and Linux machines
Arial and Helvetica, just in case, to catch any stragglers.
I got the inspiration from real word style.
Everything after the first two is basically disaster-planning to ensure the site remains good-looking or at least somewhat legible on various kooky set-ups. The first two are my personal opinions and Verdana is probably interchangeable here with Trebuchet.
I always add Lucida Grande first as I think it's a bit of a shame to have Mac OS X users reading using a font expressly designed to overcome a technical shortcoming that their system doesn't suffer from. Actually, thinking about it now, I'd probably promote Vera Sans above Verdana/Trebuchet for similar reasons if legibility was more important than consistency.