The team discussed this a while back and we never came to a consensus, but an idea the majority was fond of to eliminate this discrepancy was thus:
Allow the BoS in DC to, on occasion, have T-51 instead of T-45. Make the Fort Constantine power armor a unique in some manner (Prototype T-53a, or T-51d, or something like that). The T-51 found in Operation Anchorage is already somewhat unique because it is winterized.
There were some problems with this (which I only remember vaguely, so bear with me), namely, there is lore documentation stating that the DC BoS found large amounts of T-45 under the pentagon and for some reason ran out of T-51 (again, going off memory, which is fuzzy in this regard).
A possible solution to that would be to, rather than giving the BoS T-51, instead allow low-level Enclave to spawn with it.
A mod to remove T-51 from NV wouldn't be especially hard, but TTW will never make that change because we have vowed to make as few changes as possible to the NV side of things.