In MD, the county tax is a percentage of the Maryland state tax, BEFORE you apply the tax credit for out of state payments. So while MD (sate), does not tax you on out of state earnings, the counties do.
OK, that is much different from what we do up here in NYC/NY.
So if they calculated the county tax completely from scratch, instead of using a percentage of the Maryland tax, this would not be an issue because your out-of-state credit would be already ‘used up’ on your Maryland tax.