Normally we're able to hide the spaces where blocked ads would go. On some web pages, however, you may see them as blank areas on the page. The reason those spaces are there is that we weren't able to reformat the page to use that empty space―and the reason for that can vary from page to page. Ultimately, something about the page is different from the majority of web pages we know how to reformat.
There are millions of ways to design a website, and we try to cover as many exceptions as possible in AdBlock. But even if we could somehow manage to allow for every possible design choice, sites change constantly. We simply couldn't keep up, and if we tried to, it would be impossible to maintain.
If you start seeing blank spaces everywhere, please let us know. Otherwise, we hope the occasional blank space won't be too bothersome as you browse.