#15 is caused by a difference in gradient between the red a blue-ish triangles. If you actually check, they do not have the same gradient and when switched causes a small bulge (look closely) at the join, this is enough area to make up a single square :P Sorry for nerding it up :D

I was thinking about that the bottom line is a primer number (13) and you are trying to complete it with 2 numbers which mathematically is impossible and there is where the inconsistency shows up... but I am not sure how to explain that

I was thinking about that the bottom line is a primer number (13) and you are trying to complete it with 2 numbers which mathematically is impossible and there is where the inconsistency shows up... but I am not sure how to explain that

a-b=0

2(a-b)=a-b

2(0)=0

0=0