one or two thunderbolt displays iMac

I'm a bit confused here..

It states here: Thunderbolt ports and displays: Frequently asked questions (FAQ) - Apple Support, that an iMac can support two thunderbolt displays.

However, in the iMac specs, http://www.apple.com/imac/specs/, it sais it supports up to a 30 inch 2560*1600 display with full native ress..

So, what's the catch with using two thunderbolt displays? Do I lose the native capability of the iMac display, or why don't the specs say two displays with 2560*1600 resolution?

Also, in what way does the size of the display matter? Shouldn't it only depend on the resolution(s)?