I'm either going crazy or something here isn't right...

I'm attempting to use a ring sizing chart to figure out what size wedding band I need...

Now, I cut a peice of paper to the perfect length and am now trying to measure it...

I first measured it in inches, and it's EXACTLY 2 5/8 in... Well, according to this size chart that is a size 10 1/2...

So I then measure the EXACT SAME PIECE OF PAPER using the mm side, and get exactly 67mm... So that, according to the SAME CHART is a size 12 1/2...

So, thiking my ruler is screwed up I head over to WWW Unit Converter and convert 2 5/8 in (2.625 in) to mm and get 66.675 mm... Which basically means my ruler is 100% correct, and according to the size chart if I decide to use inches or mm I will get 2 different sizes for the same finger...

Am I going crazy here???

I checked other charts online as well, and they pretty much all say the same thing... Why must this be so stupidly difficult, as to not even make sense???