it’s because the anniversary 10% xp buff, just select the race-appropriate starting zone and you’re good.
you also wanna stop xp right after you levelled, i remember something like if you stopped xp while being in the first 10% of the xp-bar, the mobs were still 1 lvl lower (so for example if you are lvl 11 and have like 5% xp towards lvl 12, the mobs are still lvl 10), but they could’ve changed it since then.