I cannot find where, but I read in another thread that the level needed to repair a weapon is at the moment related to the value of the weapon. This is the value an NPC would pay you, not the actual usefulness of the weapon (related to slash and such). That is why a silverweave short sword (/10?) can be repaired at level 2, but a frosty short sword (/4) takes level 12 (and then it still takes 675 seconds to increase the quality by 5 points or so). I guess a Scruffy Small Battle Axe of the Warrior will make even more money when you sell it.
I think the /repair command is still being worked on, so hopefully in the future the level requirements will make more sense.
edit: typos