It has a lot to do with density, like, a lot. When you're speed farming Empowered 40-55 to level gems for Caldesann's, pay attention to the clear times when you close. They might average around 2:30, but you might see some that are 1:45 (really really great 1 floor rifts) and some that are 4 minutes (really low density Act 5 city street maps that go on for 4 floors).

You have to keep in mind that each GR difficulty increases mob health by 17%. A 13:00 clear at GR difficulty X (let's say 71), that's 780 seconds. 780 x 1.17 = 912.6, or 15:12.6 . If the rift is exactly the same but it takes 17% longer to finish, a 13 minute clear is suddenly over 15 minutes. You might get a 13 minute 71, and an over-15 minute 72, and that can be expected.

You won't find any rifts that are exactly the same, and not all mobs will take 17% more time to kill, but that just illustrates how much 17% is - one GR level increase when you're almost at your limit can push it a hair over your limit.

Those 2 things said, you might just end up having to fish for those super awesome rifts or at least rifts that are more awesome than the typical average rift - like those ones I mentioned that you see when you speed farm that are 30% faster than your average speed farm clear times. Might take a while, especially if you were really close to 15 minutes on your 71.

And if you're finding that survivability isn't a huge issue in terms of kicking it up by one level, and you're ending up clearing it 17% slower, that's not a bad thing, that's spot on. If that's the case, don't get frustrated and think your playstyle needs improvement - you can try to improve it, but remember that the way you played to clear your 71 is the benchmark. It works. You might try stuff that makes your clear time worse or gets you killed (trying to Vault way ahead to build monster packs and getting overwhelmed among other things), don't get led astray by thinking your playstyle is bad - you might end up making it worse. Try to change it but remember how you played to clear that 71.

And when push comes to shove, if survivability isn't an issue, sacrifice that toughness and stack that damage just so long as it isn't causing any excess deaths. I'm playing M6 without Awareness and with Reaper's Wraps when I'm trying to push 74 because losing Wraps of Clarity isn't getting me killed and I'm not dying much at all without Awareness. If you can fit any more damage into your build without dying and without making you play defensively and slowly, it'll straight up drop your clear times.