Are you sure about that? I have at least three anecdotes that seem to directly contradict that claim, and while I realize the plural of anecdote isn't data, but it's still enough to make me skeptical of your claim.
It may be that what I was told was correct, but they were referring only to Seattle and not places like SF where cost of living is very high.
Two other situations involved coworkers transferring teams from Seattle, one to Toronto and one to India, both had COL adjustments to their salary.
(Admittedly the evidence is less strong in the latter two cases because they involved simultaneous country changes)