In the revision-slider each revision is shown as a grey bar. Not selected revisions are light grey while revisions within the selected range are darker gray. Revisions of a user that has been selected are also shown darker grey. These two grays are similar and easy to confuse.
In the following screenshot it might look like user "Aka" not only did the second-last edit and the one edit in the middle but also the three edits at the beginning:
Suggestions: change the colors to be more distinct.