There's often no slack from the middle of a band to absorb any of that 15%. Most people valued their house for property tax at the top of the lowest reasonable band.
Possibly if the bands had been 275-325, instead of 250-300 then the range that people mentally value their property might have fitted better with the tax bands. I assume more people like to think their house is worth around 200k, 350k, 400k etc, than around the actual midpoints of the tax bands such as 375k or 325k.
It now seems you've got to pick the top of your range if you intend to sell within 3 years to minimize future property tax problems, if you think your house is worth 280-320k you better pick the 320 band even though you know if forced to sell now you're unlikely to get that figure.