Personal preference / gear.
For instance when I reforge mastery > haste >>> crit I end up with something like 19k+ mastery and 12k+ haste, personally I never bother with the breakpoints these days as they don't really do anything for your dps because of pandemic and the way we snapshot these days.
If you really want to go for a breakpoint just go for whichever one you end up closest to
https://docs.google.com/spreadsheet/...ZmFYa0E#gid=12
I've never figured out why people obsess about the 9778 and 13737 BPs as they're both just arbitrary ones and there's a bunch of them in-between. For instance UA has one at 12.4k, and agony has one at 13.1k. All the breakpoints are too far apart for you to really see a filler up time benefit for going for any specific one, I imagine people are just focusing on the highest ones before major leaps in haste but in all honesty it really doesn't matter. Just go with whatever one floats your boat since you'll be putting dots up with random meta gem procs as well as unbuffed ones and then snapshotting procs randomly with RPPM on top of pandemic working the way it does it really doesn't matter which BP you end up at.