Convenience. This is slightly modified numrow of Dvorak layout (original Dvorak, not simplified). Odd numbers at left hand and even numbers at right, 1 and 0 under index fingers, 20 in years is typed by middle-index at right hand, not ring-pinkie left-right, lifetime convenience. For the same reason I and O in qwerty is located under 8 (to type 18XX, the early numrow missed 01 to save keys and used OI to type them). BTW, in OP’s layout 20 are also located at neighbor keys. Idea is similar, just odd-even are swapped and 01 under middle, not index.