It's because at some point, about a year ago, I had "cancelled learning" on many of my verses (but I didn't reset the scores back to zero) Now I'm in the process of re-adding those verses and as I review them, many stay at 100%, some move from 96 or 97 to 100, and others fall from 100% to some lower level if I can't remember them well enough :)
Do you mean cancelling learning? When the learning is cancelled, the progress percentage does not change. So I didn't benefit from it. It's just that at the time, about a year ago, I wanted to shorten my review queue. But it wasn't the best idea for me personally with respect to those particular verses, because by not reviewing them for a year, now some I don't remember very well. I hope it's understandable.
But if you mean it's a great idea to shorten your review queue by cancelling learning on some verses, yes that might be a good idea for you, if you think so :)