XAResource.isSameRM is described withing "3.4.9 Identifying Resource Manager Instance" of JTA spec.
Discussing this with Jonathan Halliday a cluster itself should be seen as a ResourceManager, which means that the semantic of XAResource.isSameRM(XAResource) is as follows:

• if it is the same type as ours (TransactionXAResource)
  AND
• other's cluster name is same as ours
  then return true. False in all other situations.

Current implementation returns true if and only of the two TransactionXAResource implementations are associated with the same Transaction object.