The technical reason its currently possible:
We have 5 cycles per day of 40 blocks.  In each cycle, when difficulty drops to 0, an inrush of people tithe and cause diff to be outside its bounds (IE Too high).
This happened every cycle yesterday.

I'm working on a fix as highest priority.  This is actually a small fix but will require a mandatory.
We can make this algorithm work perfectly.

Please be patient everyone.  In the mean time, don't be greedy and tithe too much at once, or everyone will receive negative ROI that day.