That's odd, using the separate names for the same thing.
Not as odd as you think. Three examples just off the top of my head:

Cumming/Buchan District

Campbell of Cawdor/Argyll District

Murphy/Tara (Ireland) District

That's not all, I'm sure! :mrgreen:

Sometimes a clan/family tartan is chosen as a district tartan (as in the case of the Cumming/Buchan) because that particular family is/was prominent in that district.

Further reason, IMHO, for a centralised tartan register to record all of these variants and end some confusion.

Cheers,

Todd