The target EPR has to be set with ref to some speed or other, at a given thrust setting it will be varied by speed. 80kts is the speed chosen to be fast enough that there's time to stabilise parameters and not so fast that a shortfall will cause a high-speed RTO.
As airspeed increases with throttles set the ram effect will vary EPR. (Increases, presumably). Modern, clever types (eg767) have a/t submodes which engage at 80kts (throttle hold)which prevent low level inversions etc from causing throttle movements for a period of time after t/o.
Most commercial a/c I've flown have had EPR gauges and I HATE THEM!!!!!!!!!!