Level costs depend on where you put items in an Anvil

I think that's intentional.

I don't get it: In Both cases you'll get a sword with KnockBack II, Fire II and Sharpness IV.

Please describe your issue better.

After help from other mods and re-re-reading your descripttion I guess that you're referring to the different costs of 30 to 34 levels.
That's working as intended, read on here: http://www.minecraftwiki.net/wiki/Anvil_mechanics
For example there is a penalty for already repaired items, that's the difference for putting them in the left or right slot. The resulting items is always the item in the left slot.

Kumasasa: The minecraft wiki describes the game as it is. That the wiki describes something doesn't seem to me to be great evidence that it was intended, or that all of the implications have been considered by Mojang.

I don't think this is intended. There is no reason the different slots should work differently.

Again: If the item in the left slot was already modified (renamed, combined) on the anvil, then there is a penalty for having used the anvil. If the item was never used on the anvil, then there is no penalty. The resulting item is in fact the same regarding the enchantments, but not in initial penalty.
That feature is quite useful to keep weapons over a longer time.

What? My complaint isn't about how long an item will last, it's that there are two different behaviors depending on which slot you put the items in. This is pointless, and should be fixed. It should just always give you the lower penalty, no matter how the items are arranged.

It's explicitely coded the way it is, any change to it would be a feature request.

So why is this considered intended, while MC-75474 is still open? They are both cases of something happening differently depending on the order of player actions, even though there is no reason for it.