I agree that a lot of the themes on here use or must use very similar HTML markup because the layout of one theme very much resembles the next one. oh they use different content and fancier graphics or jquery functions on the next one down the list but sure look similar to me.
if you want to see what can be donw with the same HTML markup used for all the example web pages, look at the web site called csszengarden.com. all of the pages on display in this site use the same HTML markup, each author has used his/her own version of CSS and images but the intent of the site very simly displays that different looking web pages can be created using the same HTML code.
interesting site but a little dated.