I'm curious because in places such as Cleveland, a lot of houses lost value, but most not very much. That's because there was never a housing bubble here. The only exception is inner city neighborhoods that self-destructed.
It just says “created negative returns for homeowners,” without detailing (in the excerpt ;-) exactly what that means. It could mean they looked at each transaction, identified the gain/loss, and then averaged the amounts. That would take into account the magnitude: fewer large losses and more small gains could still give you a “net loss” period.
On the other hand, they could have looked at each transaction, marked it (+) or (-), and chosen the direction with most transactions. That could not recognize the amounts of gains/losses.
My immediate area had a similar result to what you describe in Cleveland over the last decade: a period of a few years in which there were modest losses as a percentage of total price.