That it's the same thing may be more familiar in base 10: for example with 3-digit numbers, the 10's complement of (say) 428 is 1000-428 = 572, and you get the

That it's the same thing may be more familiar in base 10: for example with 3-digit numbers, the 10's complement of (say) 428 is 1000-428 = 572, and you get the same result if you "invert" each digit (subtract from 9) and add 1: 571 + 1 = 572. This is just because (999-x)+1 = 1000-x.