If you read deep enough through Pinterest's API documentation or happen to hit their "Get available metrics' definitions" endpoint, you will notice that Pinterest provides two different metrics for paid impressions, one named "PAID_IMPRESSION", and another named "IMPRESSION_1". The definition for both of those also happens to be fairly similar.
"IMPRESSION_1": "The number of times your ad was seen."
"PAID_IMPRESSION": "The number of impressions that you paid for."
It seems the only difference may be whether Pinterest considers the pin or ad as something which you specifically targeted with your payment. Pinterest then goes on to explain here that they append "_1" to a metric to identify it as "first order ad events" and "_2" to a metric to identify it as "downstream, earned events".
So...that is all pretty confusing...what is the appropriate metric to use? We answered that question by looking at the data. Through our research, we found that "IMPRESSION_1" happens to provide paid impression data at times when "PAID_IMPRESSION" does not, so that provides a conclusive answer: if you want to have paid impression data, you should use "IMPRESSION_1" (...and we do, because we prefer to provide data instead of null values).