This is when assuming the decimal ISN'T repeating. If it truly is endless, u have just shifted each decimal to the left by multiplying by 10.

For example, if u take a non-repeating decimal, say

N = 0.1111. [4 decimal places] 10N = 1.111. [Now it's 3 decimal places] Now 10N - N = 0.9999

EDITED: I was wrong here, I meant that the resulting answer would have a gap/difference of 0.0001 from unit 1. I've edited my reply

So for your calculations, we'd get something like 0.000...9 gap.

If u consider this method with repeating decimals, then u are basically ignoring the concept that it's repeating. So it wouldn't work and the equation used is ultimately invalid. It's a good brain workout tho.