ThePrincipal Residence Exemption (PRE) sets lower property tax rates for property that is the principal residence of the owner. So rentals, for example, are generally taxed at the higher rate.  See the city's Property tax history for rates.  The state allows districts to levy up to 18 mills on non-PRE properties, and AAPS levies close to that.  (TODO: I'm not sure why it isn't exactly 18 mills.)  Ann Arbor also levies about another 4 mills on PRE properties, allowed by its status as a hold-harmless district (TODO: sounds like it could levy more, I wonder why not?).  In addition, the state collects a 6 mill education tax on all PRE and non-PRE properties, and WISD collects about 5 more mills.

Further reading:

The Basics of School Funding


mlive: washtenaw school taxes


WDET: how are michigan public schools funded