It has never been officially confirmed that the other 10 potentially coming are double deckers. They seem to have gone out of their way to not say it in such a way as "another 10 are coming". The single deckers would be a lot more affordable with Aberdeen Council not having to pay a contribution for these 10.
You beat me to replying but you're right, some articles last year did state and allude that the further 10 would be double deckers but the wording was generally quite vague and the specific type has not been confirmed. All we really do know is another 10 are coming, they're from Wrightbus and intended for the 23.
The one thing that's always peed me off a bit with First Aberdeen is branding buses for a particular route then not sticking with those routes.
Even before Covid there were many examples of branded buses running on different route numbers.
Maybe it's a virgo pedantic thing and I know there are breakdowns etc. but it seems they got to a point where the branding was ignored. Why spend all that money and time on the special livery for the buses to be allocated anywhere?
The issue Aberdeen have had up until recently is that too many and often of the wrong type were branded. For example they had 15 Enviro500s branded for the 1/2. However on weekdays when the Uni Closed timetable is in place the 1/2 only required 17 buses and they'd often allocate 10 or so Artics on the 1/2 meaning that half of the branded E500s were surplus and ended up being used on other services. With repaints progressing fast the intention is to brand less E500s this time (possibly only 7 or 8) that way the problem should in theory be resolved.
Likewise with the 19, with AWPR opening making journey times faster and the frequency being reduced from 12 to 15 minutes the vehicle requirement of the 19 off peak went from 12 to 10, at the time this happened Aberdeen had 13 buses branded for the 19. 69357 was repainted a few months later but for around 2 years Aberdeen had 12 branded Geminis to cover 10 duties, there was an 11th duty that just operated in the morning peak however this was usually single decker operated.
The other reason is because of interworking, as an example two daytime buses on the 13 until recently used to worked on the 3 and 8 in the morning, while another works on to the 12 in the afternoon.