Attribution models divide credit for conversions that occurred. Incrementality testing asks a different question entirely, which is how many of those conversions would not have happened without the spending.
Attribution assumes the conversion needed a cause
Every attribution model starts from a completed conversion and allocates it among the interactions preceding it, whether by first touch, last touch or some distribution between them.
None of them can consider the possibility that the purchase would have occurred with no advertising at all, because that case leaves no touchpoint to examine.
Channels that appear near existing demand therefore accumulate credit efficiently, which is why retargeting and branded search consistently look strong in these reports.
Incrementality requires a group that was not exposed
The method holds back a portion of the addressable audience, or a set of regions, and compares outcomes against the exposed group.
The difference between the two is the effect attributable to the advertising, measured without needing to observe any individual's path.
This sidesteps the tracking problems that undermine attribution, because the comparison works on aggregate outcomes rather than on linked records.
Results often contradict the attribution report
Channels that dominate attributed conversions frequently show modest incremental effect, since they were reaching people already intending to buy.
Channels that attribution barely credits sometimes show substantial incremental effect, because their contribution occurs long before any measurable click.
The disagreement is not a fault in either method. They are answering different questions, and only one of them is about whether to keep spending.
The tests have real costs
Holding back audience means forgoing revenue from that group for the duration, which is a genuine expense rather than an accounting one.
Tests also need enough volume and enough time to distinguish an effect from ordinary variation, which puts them out of reach for small campaigns.
Running them on the largest lines of spending, where the stakes justify the cost, is the usual compromise.
The two methods work together
Attribution provides daily feedback for operational decisions such as pausing a weak creative or shifting bids, which incrementality testing cannot supply because a single test runs over weeks.
Incrementality provides the calibration that tells you how much of that attributed performance to believe, expressed as a ratio that can be applied to the faster reporting between tests.
Using attribution for steering and incrementality for budgeting keeps each method inside the range where its assumptions hold, and it stops a channel from being defunded on evidence the method was never able to produce.