The warning is:
This server type expects a version of 6.1 but the server directory is of version 7.0.0.GA. This combination may cause critical errors.
I guess it's because ER6 reports version "7.0.0.GA".
For example ER2 doesn't raise the warning (because it still reports as 7.0.0.Beta1).
Legacy product name in server log and CLI
JBoss-Product-Release-Name has changed in eap manifest, breaking tools expectations