In WoW, stuff scales (mostly) to match your level. If you overgear a low level toon, your damage skyrockets, but as you level, if you don't update your gear, you wont be nearly as effective. Some who farm specific things (or pvp for that matter) will lock their exp at a lower lvl to fit into that bracket. Heck, some casters basically use zero mana with spells for a bit after starting, so on top of no mana use, they effectively 1-2 shot enemies. Enchant gear, and you have a lvl 1 wrecking ball. Pretty sure there's a minimum lvl 10 to shut off exp tho it may have changed. When you party synch, or use the Dungeon Finder for a random group, the monsters scale for the entire group. So a lvl 70 in the same party as a lower lvl will end up killing slower while the low lvl can just poke it and it fkn dies.