On March 19th (or 18th*) YPN reports started showing my ad impressions down by about 20% on my two small sites runnng YPN. The YPN impressions reduction has been consistent ever since.
Meanwhile, both sites traffic has remained stable -- as they have mostly been for several years.
My belief is that YPN has simply stopped displaying ads for untargeted countries (or stopped counting them). One of the two sites has a portion of its traffic from India -- about enough to account for the report differerence.
Is anyone else seeing this or know of some other explanation?
(*) I briefly added a larger website to YPN for about 2 hours on the 18th so I have mixed-up numbers for that date from YPN.