Upgrade stats unbalanced. Aether?

So I just wanna ask about the upgrade stats.

Comparing theese perks: farmer, xp, gold, luck and skull bonus. As we upgrade them at blacksmith they all increase by a incredible 0.1%.

This is understandable for skull bonus since a little number here goes a decent way and the Base stat is low. Highest I got was 2.7 on a level 110 item.

But why is it the same value for the other perks I mentioned? With xp boost as a comparison I got a 18.6% value as my belt became uber and it only increases by 0.1 why would anyone bother that? If I make the skull bonus item +20 I almost double the Base stat I was given when it became uber. But if I do this with the xp item it becomes a totally unnoticeable difference for the same price.

Is this just sloppy coding for all items that has a % value. Or just completely overlooked?