Oh, dear God........
How does THAT happen?
Well; if you do the math Tan(X)=2/5 and Tan(Y)=3/8 you'll find that X and Y do NOT equal each other.
Close; but no banana.
Print out the picture; sight down the hypotenuse; and you'll see the angle difference.
Google® 'the missing square' and a more detailed account can be found.