See my reply to Mallika above.
For 999 the digit sum is 9 or 0.
There are many other times that we get 2 or more answers to a mathematics question. For example in solving quadratic equations we generally get 2 solutions.
There will never be any contradiction in using either 9 or 0. Using either will give the same answer. It’s like saying the time is 1pm, or the time is 13:00 hours – they are equivalent.