Hits above 99% have a bug that only allows people with 100% accuracy to take them, but it's only true 100% accuracy. That means that if you've ever gotten a hit rejected but have worked your way back up to 100% you'll still be unable to take it because technically you really don't have 100% but instead something like 99.99999997
I've been Turking for a year and a half and it's been this way ever since I started, so I wouldn't hold your breath for it to change.
The approval rate qualifications math is dumb. >95 actually means 96.0 and up (i.e. 95.9 isn't good enough). This isn't documented anywhere, so a lot of requesters who want people with 99.0-100.0 will set greater than 99, and not realize they're requiring 100.
It may be possible to round up to 100 by getting 99.95. I think amazon rounds the rating. For example, my rating is 99.85, but my dashboard says 99.9. But yeah, the qualifications work with whole numbers, not decimals, so >99 I think you would at least need an approval rating of 99.95.