@Deeogee: A style isn't just a simple page constructed from XHTML, CSS, and images. It also contains template variables, switches, language variables, etc. which can and do contain bugs.
The reason why there's so many changes is that there may be errors within the template syntax that can cause unexpected behavior. There's also design bugs and other adjustments being made. We require most, if not all of these changes if the style submitted contains the exact (or almost) files from one of the default styles. This is to ensure that the styles do not contain bugs that were fixed several releases before. It will also decrease the amount of support requests that a certain style has. If a new feature is added to a new release, it will more than likely require style changes in order to add the interface for that certain feature. Styles that are not updated will lack that certain feature, and we'll soon see users start asking why isn't feature X working in style Y.
Also, the purpose of the Styles Database is to have styles that work properly with the latest version of phpBB. There isn't much point in releasing styles that don't fully work.