The measure is generous to keep things moving
It gets corrected later, at the point in the job with nothing left to absorb it.
What's happening?
A generous interim measure feels harmless: the money is early, not extra; relations stay warm; the certificate goes out on time. The debt comes due later, because over-measure has to unwind - and it unwinds at the exact end of the job where there is nothing left to be generous with: the last applications shrink, cash tightens against demobilisation, and the final account opens with an argument about clawback instead of a reconciliation.
The alternative is not meanness, it is a build-up: the measure evidenced item by item - quantities, records, basis - so this month's number is this month's work, defensible in both directions. The build-up template holds that shape application by application; where the measure itself needs doing properly each cycle, the valuations service takes on the whole routine.
The solution
Measured once, evidenced, and never borrowed from later.
If you would rather it were done for you, Interim Valuations is the service that does it.