I've seen two general explanations. I'll present them without comment, though I recognize there is plenty of room for that on both: 1) it's considered easier t

I've seen two general explanations. I'll present them without comment, though I recognize there is plenty of room for that on both: 1) it's considered easier to justify internally, since the worker in the more expensive area is being paid at a closer to market rate for their market (rather than above rates in their market); or 2) the management of the organization is usually located in the more expensive area and internally evaluates rates there as the baseline (and the remote rate(s) as the negotiated special cases).