No, different styles can be different because there are variables that get inserted into the HTML and you can change where. Also, there are SGML entities that vB interprets as conditionals; for example the postbit relayout for the large avatar is controlled by something like this:
For the most part, though, you don't have to worry about the conditionals if you don't want to. It's fairly easy to figure out why most of them are there and you can just change the HTML arround them.

I've given you style admin on the test board. Please only change stuff in the "Lenny" style.

The manual is here
http://www.vbulletin.com/docs/html/
You should probably read the "Styles&Templates" section.