All creative types use delayed impressions. This means that impressions aren't counted until the creative renders and sends an impression request back to Google. Because of this secondary request, RTB breakout and RTB Troubleshooting can return a greater number of winning bids than billable impressions.
Expiration windows
The expiration window for impressions-based billing events varies by inventory
type. The expiration window of the request in seconds is sent in the
BidRequest.imp.exp field in OpenRTB, and
BidRequest.adslot.impression_expiration_seconds in the deprecated Google RTB
protocol.