I'm not sure what exactly you are trying to argue here.
First off I totally agree the percentage thing confuses things here. Also apologies if this sounds like I am talking down to you or being patronising. This is not my intent whatsoever. I am just trying to clarify things from my own understanding.
I am not sure how they work out the whole percentage thing. It 'may' be similar to how we convert fractions to percentages(I don't really know). But, from what I can see in your picture you needed to roll a total of at least 11(based on the AC) to actually hit them. In the breakdown you only rolled a total of 10 so rightly so that would 'miss' an AC of 11.
The maths(at its most basic) would be you need to roll a natural 5 minimum to hit. This would mean you have a 15/20 chance to hit(or 75% chance). But because you have advantage you get to roll twice. So this would obviously push the chance to hit up. I don't know what the percentage would be(my maths knowledge isn't that good sorry) but obviously higher than 75%. I am guessing the game only showed the higher roll for some reason(the l;ower roll being redundant).
I hope this helps.
