How can a portfolio have higher revenue model coverage than cost model coverage?

It's possible for a portfolio to have higher revenue model coverage than cost model coverage in either of the following scenarios:

• One or more of the bid units received revenue in the past and have revenue models, but they have received little or no traffic recently and therefore have no cost models. This type of bid unit is currently considered "zero impression" and is not expected to spend any money at any current bid level.
• One or more of the bid units haven't received any revenue and have received little or no traffic recently, so their revenue models are based on revenue trends for the advertiser's other, similar bid units, which may have received revenue.