It does. This is definitely something that's being actively iterated on though. Let's say as a reviewer you leave some comments on Revision 3, then the user pushes 2 more PR revisions and asks you to re-review. The default diff when you open the PR again will be from Revision 3 -> Current code. It lets reviewers easily see what's changed since their last review.

There are also the "Mark as reviewed" checkboxes per file that can help you track what's been looked at.