Makes sense if you look at each column, comparing left and right.
Highest order column = One 9 on left, nine 1's on right.
Next : Two 8's on left, eight 2's on right.
Next : Three 7's on left, seven 3's on right.
etc.
It may or may not be related mathematically - but when I was a kid daydreaming and messing with a calculator in math class, I figured out that if you add 123456789 and 987654321 the answer is a bunch of 1's. If you change the pattern and snake around the number pad - and always reverse the pattern - you still get 1's. e.g.: 321654987 + 789456123 = a bunch of 1s. I know it's not rocket science - but as a kid I thought I was frickin Einstein.
Related concept In base N (in our case, N = 10, because humans have ten fingers), N-1 scenarios analogous to these yield the same results.