Watching the interest bar on my ad I have noticed that while mine is going down other ads are increasing thier interest bar.
Also my interest bar appears at different levels when viewed on different pc's around the office. I can't think of a clear reason for this?
Think of the Interest bar as a graphic representation of the CTR of that ad for the keyword searched.
Essentially, if the Interest bar is full, it means that the CTR is at or above 1% for that ad/keyword.
The interest bar can change from page-view to page-view (therefore from computer to computer), because the CTR of the ad/keyword also changes from moment to moment.