I am afraid that cannot be the difference since I am MP IR rated as many others are, or maybe it is the way you say but my national civil av authority misinterprets the rules.
on the other hand when reading appendix 9 to part FCL which describes atpl flight check it reads "The test/check should be accomplished under IFR, if the IR rating is included, .....", as far as I can understand it means that if you have already got your IR rating there is no need for you to accomplish that part of the check (which I think corresponds to section 5 out of the 6 possible sections within).
thanks anyway