The RX/R designation doesn't actually say much about the underlying architecture, Polaris is GCN 4, while HD 7000 is ... a mixture. Most HD 7000 cards are first gen GCN, though some are TeraScale 1/2, which would required very different driver support.

I guess it doesn't "matter" to consumers buying a product, but it is unfortunate how confusing the naming schemes are versus the underlying architecture. The R9/R8 cards were similarly a mixture of GCN 1, 2, 3, and even TeraScale.

(similarly, the RX 5xx/4xx series has GCN 1 and GCN 3 cards mixed in, in addition to the actual GCN 4 chips)