One versus the Other One

How come in one realm, someone could be so great but in another realm not so hot? Is it because where they are great, they are surrounded by people they have raided with them for so long that it doesn't matter how good you are? In my opinion I play a pretty decent healer, but I just can't take it when someone says I can't.....it takes almost everything I got to not let them "accidently" die.There are only so many classes that can heal, so I play a 2nd toon of a druid heals in two different realms. Both are even in specs, vary slightly in gear, but apparently can't heal in the other realm.