Good morning, 
Thank you for the question.
When adding the building polygons to the area, there were existing address nodes (as mentioned in my changeset comment) from a previous import. This tag was added in that original import to most nodes that indicated more than one residence in a building. As I merged, I was cleaning up, to the best of my ability, the tags that were irrelevant as well as clarified the addr:unit=* tag to only include the numbers/letters of the unit. A couple of the tags slipped through it appears, but I've gone through and removed the handful in Newport Beach now. 
For more information on the original address node import, you can visit that wiki page here: https://wiki.openstreetmap.org/wiki/Orange_County_Building_and_Address_Import . There is a bit about how they translated the tags from the source to tags applicable to OSM that touches on the appearance of the addr:unit=Apartment tag. 
-Arianna