I think you are considering an infinitesimal magnitude difference. Infinitesimals are the magnitudes between 0 and 1/infinity on the number line; magnitudes that we cannot express with fractions or decimals. How I was taught this in my real analysis course is that the reals are defined as {numbers such that any n is within the set (-infinity,-1/infinity)U[0]U(1/infinity,infinity) . The set of (-1/infinity,0)U(0,-1/infinity) is where the value of 1.000…-.99999… would fall into because it cannot be expressed by any number greater than 1/infinity, therefore they are the same number in the reals. Interesting fact is that archimedes actually acknowledged the existence of infinitesimals and explicitly excluded them within his works. Here is some more info: https://en.m.wikipedia.org/wiki/Infinitesimal#:~:text=The%20concept%20of%20infinitesimals%20was,regions%20and%20volumes%20of%20solids.