We assume that it is 1 because it makes calculations way easier, and the error it creates in reality is so little we can't notice it.
Maybe in the future it will cause problems.
We know .9999... doesn't =1
but in math we say this to cut down the paper work.
There's many other things in math that we assume or bend to our advantage.
so literaly .9999999 doesn't = 1
but for the sake of saving time we say it does.