That’s just 0.6 and 0.0 written cleverly.

If the 9’s repeat forever then there is no possible number between that and 0.6, so it must be the same…

Put another way, any single digit “n” repeating is just n/9.

Your problem is equivalent to (4/9 + 5/9 - 0.4) + 0/9

(Equivalent to 0.6)

You don't know if those 9s repeat forever in this problem. It's output from a program, and the program could switch to 3s in 1000 more digits. The "adding" program is reading text digits from two other programs and can't see how they work. It can't assume a bunch of 9s mean there will be more 9s.