This is the timeline of the events:
- Go to Underground Ruins in Clay Mountain. The ring does not have a color.
- Press the switch and the ring turns red.
- Go to the chamber and catch Regirock.
- You are given the Iceberg Key if you have White 2 and the Iron Key if you have Black 2.
- Go to main menu. Select Unova Link, then Key System and then Key System Settings. Then Select the Iceberg Key/Iron Key.
- Go to the game again. You'll see that the ring color is now changed to white/gray. Go to the chamber again and you get Regice/Registeel.
As for your question. There is no way you could have caught Registeel without having the Iron Key first. So if you have transferred that from another game and selected it, then the chamber will give you Registeel and not Regirock. What you need to do is go to main menu, select Unova Link, then Key System, Key System Settings and then select Rock Peak Chamber. Then go in game and press the switch. The ring color will turn red. Then go to the chamber and you get Regirock.
If you didn't have an Iron Key and the game gave you Registeel, then it's a glitch or something and I highly doubt that.