The residential exemption is a trailing indicator that understates owner occupancy, especially in new buildings.
From the City's website:
Residents
began moving into MT in July 2016, but most units weren't first occupied until months later. However, in order to be eligible for a residential exemption for a given address in a given fiscal year you had to live at that address on
January 1st of the previous year.
It was thus impossible to get a residential exemption for any unit in MT for FY2016 or FY2017, as nobody lived in the building on January 1, 2016. In order to be eligible for a FY2018 exemption you must have lived in MT on January 1, 2017 (i.e., you moved in within <six months of the first unit opening). If you moved in to any unit after 1/1/17 you won't be eligible for an exemption until FY2019.
And even if you did move in before 1/1/17, residential exemptions for 2018 are still being processed (the deadline to file was just a couple weeks ago).
In short, it is entirely possible that ~80% of MT units are primary residences while only 25% have current residential exemptions.