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Even ADE does not fully conform to the EPUB spec in regard to rendering. ADE even has it own "extensions" to EPUB that Adobe thought it good to add.
That said there are two issues here. The EPUB spec is based on XHTML which is large and complex. Implementing every frature is a daunting task and not something that can be done quickly. Especially when taking into considerations the limitations of the reading device. Such as amount of memory, speed of processor, color screen.
The next issue relates directly to the content using XHTML. The spec isn't very clear in certain cases about how to render the content. This is the same reason Chrome, Firefox, IE and Opera often display the same web page differently.
Keeping this in mind calibre's viewer (Sigil too) uses the Qt WebKit component for rendering the book. Opening the same book in the Firefox addon can give a slightly different look. Who is correct in this case? Both readers conform to the spec and both are massive projects to display that book.
That said displaying a blue gif as red is wrong and centering a heading just because is wrong too. However, your never going to have a book look exactly the same in every reader if you use complex formatting because the EPUB spec can be interperted differenty by different people. Also, because some features are hard to implement and good enough is good enough.
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