Very interesting. That's a big gap.
There is some reserved space (5% of disk I think, to the OS) so the numbers don't always match up exactly, but 40% off is way off. Does it differ depending on what time of day you run it by chance?
A debug should have in it information about partition rotation and related checks in the manager.log. It should log when it's checking and whether it's found something to chop off. (I believe this is all at default levels, so you should see it without us increasing the debug level.) That info might tell us what's triggering.
We did for a little while also have some partition drops mis-firing - but usually it was NOT firing, not firing extra. That was resolved sometime around 6.0, so it might have been added in to mainline in 6.0.1. I'd have to check, but hopefully the logs can provide more insight.