I don't think it's anything to do with that, I think it's because the vehicles are 24m in length rather than 26m.
On the contrary - making them 125mph capable only and thereby reducing the amount of dead space needed means that they can fit more passenger accommodation into the shorter carriage length than if they'd simply scaled it down (or lopped 2m out of the middle) - Hitachi won't have gone around leaving unnecessary dead space in the existing 80x design if it wasn't necessary