Yeah, it's to do with the fact that every single browser handles tables differently. Nested tables are a pain in the arse at the best of times, so no-one wants to bother rewriting any code, so they just tweak it until they work in IE and then leave it, despite the way IE handles tables being utterly retarded (did you know it's possible to have a table occupying 100% of an area, along with another table at 10% of the area and yet more tables at say 15 pixels all coexisiting in IE? Seems stupid to me).
You want proof? My friend rewrote vBulletin so that the tables actually worked properly in Opera and Netscape... you can see them at
www.demolicious.org/forums.
That's what a sight should look like when it's actually designed to STANDARDS, and not just "the browser everyone uses".