I'm sure that for many a year, Stagecoach's night services were all 9xx, as in the westhill service was 917. I used to live in westhill and still lament the withdrawal of 215 as the main route number but that's a whole other story!
Indeed night services use to use 9xx series numbers. But back then First's night services were just numbered 901 - 905 & 920. The night version of the 17 to Dyce was numbered 904.
First Aberdeen were the first to change their night route numbers, so their N17 was the first. On top of this their 62 predates the Stagecoach 62. Many residents in the Westhill area now want Stagecoach to renumber the X17 to the 47 to make it less confusing, certainly i'd agree with them. When Stagecoach got rid of the 3 digit numbers on core services it was meant to make it easier but in reality it's made it pretty confusing.
Westhill services used to be the 215, 216 & 217
215/217: Aberdeen - Woodend - Westhill (215 and 217 took slightly different routes in Westhill)
216: Aberdeen - Lang Stracht - Westhill.
I much prefer that route system as the numbers are not confusing and the actual level of service was far better split.
Another example of where this has failed is the 107,117 & X7 which until recently were the X6, 7, 7A, 7B, 7C, 7S & X7. The 7C has been axed so there's less variations, but still pretty confusing!
It looks like Aberdeen are to see further cuts as the 220 is being axed, leaving large parts of Westhill with no bus service!