Bug with either damage calculation or tooltip error for Iron Rune

Was doing some DPS testing on my crossbow when I came across something odd with the iron rune damage calculation.

Crossbow came with 104% increased physical damage leading to an original value of 47-188 physical damage

I put one iron rune in which should set the increased physical damage to 124% leading to a value of 56-224 ( Basically some quick proportions 47/1.04 = x/1.24, if you solve for x, you get 56, same process to get 224).

HOWEVER, when I set just a 20% rune, the value shown was actually 52-206, which is actually only 114% instead of 124%. Then when I set the 40% rune in it went to 56-224.

Basically, I think the rune's tooltip might be off (I don't have a way to tell if the underlying values are wrong or whatever, it might just be a visual bug). but if that's the case, technically all the iron runes that are being used are showing a 20% decreased version (or in the extreme case, giving 20% less damage than it claims)

I didn't think to take a before and after picture so all I have is the finished crossbow right now. You'll just have to believe me that the original value shown was 47-188.

Am I missing something here? This isn't a DPS calculation so quality shouldn't affect this. No other variables were changed other than the runes being added.

I just upgraded a weapon to +20% quality and socketed 2 iron runes to have just a 38% damage increased.