Past two weeks or so, some of the pinnacles I've been getting have been dropping at either to level with the highest leveled piece of gear in that slot, or a few levels below it. I know it's not powerfuls I'm looking at since I just did 3 matches of gambit and got an 1580 heavy gl and a 1589 primary fusion while I already have a 1590 Outbreak.
Gambit, crucible and strikes pinnacles are +1, whereas other pinnacles are +2. The pinnacles drop based on your highest average PL, e.g. if your highest average is 1587 7/8, +1 would give you 1588 and +2 1589. It does not take into account what PL gear that slot already has, which can be higher.
So, based on your 1589 drop, your highest average is 1588 something, i.e. gambit +1 dropping at 1589 is correct.