Apparently my haste breakpoint formulas still aren't perfect. It's been pointed out to me that if your tick time lands precisely on the point where you'd get half an extra tick, the game sometimes rounds up and sometimes down.
In the one that's noticeable for Druids (exactly 12.5 ticks of Moonfire/IS), the game rounds down to 12. But Warlocks noticed that if they gear to exactly 7.5 ticks of Corruption