A programmatic impression is the result of a sequence that completes in well under a second. Knowing the order of the steps explains most of the problems that appear in reporting.
The request is assembled before anyone bids
When a page begins loading, the publisher's ad system builds a description of the opportunity, including the page, the format, and whatever is known about the visitor.
That description is sent to buyers as a bid request, and its completeness determines what anyone can bid on.
Requests missing identifiers or page information attract fewer and lower bids, which is why inventory quality problems often originate before the auction starts.
Buyers evaluate against their own targeting
Each buying platform matches the request against active campaigns, checking targeting, frequency state and budget pacing.
Where a campaign matches, the platform calculates a bid based on its predicted value for that specific impression.
All of this happens within a strict time limit, and any buyer who responds late is excluded regardless of what they would have bid.
Timeouts are therefore a commercial constraint as much as a technical one, since a buying platform that is consistently slow loses auctions it would otherwise have won.
The auction resolves and the winner is called back
The exchange collects the bids that arrived in time and selects a winner according to its auction rules and any publisher floor price.
The winning buyer is notified and returns the ad markup, which the browser then loads, and the creative appears.
Each of those steps adds latency, and slow creative delivery is a common reason an impression is won but never rendered.
Won is not the same as seen
Billing usually occurs when the ad renders, not when the auction is won, so a proportion of won auctions never become billable impressions.
Viewability is measured separately again, after rendering, and depends on whether the unit entered the visible area for long enough.
The gap between bids won, impressions served and impressions viewed accounts for most discrepancies between buyer and seller reporting.
Why the same impression can be offered twice
Publishers frequently offer inventory to several exchanges at once, so the same opportunity appears in multiple requests.
A buyer active on several exchanges may therefore bid against themselves for a single impression, raising the price they eventually pay.
Deduplicating supply paths is worth doing for that reason alone, before any consideration of fees further down the chain.