Perhaps it indicates that the Airport service is growing in popularity, outstripping expectations in volume growth and that, if not getting close to capacity, it soon will. After all, the Airport had 8.7m passengers in 2015 and by 2017, it was handling 9.9m. That's pretty big growth!
I think you're spot on here.
Aberdeen has had a similar situation to some extent. The Stagecoach North Scotland JET727 was started in 2010 with 3 E300s on a 30 minute frequency, these were replaced by 3 Versa's towards the end of the same year. Less than 4 years later in Spring 2014 the PVR had increased to 7 with additional Versa's being bought (12 plate, 13 plate and two 63 plates). Then by 2016 it was converted to decker operation with 8 Enviro400s, since then two further E400s have joined the fleet and the route is still seeing growth. I believe the Jet727 is actually the fastest growing bus route in Scotland, It also won "Best Bus Service" in both 2011 and 2017. In short in just 5 years it went from Every 30 Minutes with 30 seater Versa to Every 10 Minutes with deckers and is still growing, with the new exhibition centre opening in the next year or so and further expansion to the business park there i wouldn't be shocked if it gets further enhancements in frequency in the not to distance future. To say it's been a success is a understatement!
I've went very off topic here, but it's got me thinking quite a bit - so i may post in the Stagecoach NScot thread! First Glasgow are clearly having a similar sort of growth with the 500. So the move to decker operation this soon makes sense, there's no point waiting till the E200s have reached a certain age before replacing them when the growth and demand is there for decker conversion.
As for where the E200s will go, with my limited knowledge of the Glasgow network i would have suggested the 77. But perhaps they could remove the luggage racks and transfer to FSE for the 38 in the hope of growing the route. Alternatively they could move to Aberdeen and operate on the rumoured Service 4 to Portlethen which would go directly up against Stagecoach.