There is a huge difference between a -1000 resist check and an unresistable spell. Rapture, like Dictate, are not unresistable. I will use the tashan line of enchanter spells to help you understand.
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Level 54 golems in the hole are immune to magic. which is why tash lands on them but rapture will not. only a spell that is "unresistable" will land on a mob like this and rapture is not. it could have a -10,000mr check and still not land.
Basically the -1000mr check means it will land almost every time on any mob not flagged as immune to magic or unmezzable. but if they have one of these flags it wont.