You are somewhat right, matchmaking is based on the highest level of gear you have equipped. For example if you are level 30 but the highest level item you have equipped is level 22 you will fight at level 22.

have i fought you? my ign is LC at lvl 25

CombatPet is right, you will either need to relog after changing gear or do a full refresh otherwise sometimes it may not pick up that your gear changed right away.

That's interesting, you'd hope it wouldn't work the other way (change to high level gear while having a lower matchmaking level)

Oh, there's also a matchmaking range in PvP battles.
The range is +/- 3lvls.
For example, if you're lvl6 then you can be matched against lvls 3-9. That explains why you're facing lvl9 build against your lvl6 build.

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Aeacus