True, but the 18:30 return is - and that stops at Bishopbriggs too.
So, I think, currently, the cheapest option is an Anytime single from BBG to GLQ to connect with the 09:15 from there to EDB. Returning on the 18:30 direct train.
£2.10* + £14.90 = £17.00 (fares discounted with an SRN) - *I suppose if regular travel is required then a flexi would be ideal.
Probably the cheapest option, yes.
Supplementary question: why, if BBG takes its fares from GLQ, do the first class fares from GLQ to EDB not appear from BBG in the same way as the standard ones do? Would this be a deliberate action or an oversight?
It's a bit more complex than that. BBG to EDB fares are actually priced as part of the flow between Q558 and Q553 clusters. The former might roughly be described as "stations near Glasgow" and includes GLQ as well as BBG, whilst the latter is effectively "stations near Edinburgh".
However, there are override fares in place for the specific GLC/GLQ to Edinburgh flow, as you can see in
BR Fares' Expert mode. This override adds various ticket types including the Off-Peak 1st Day Return as well as Anytime and Off-Peak Returns, and reduces some fares (such as the Anytime 1st Day Return) slightly.
Fares overrides only applies between specified station pairs, so since there isn't an override for BBG to EDB you instead get the fares from the cluster-based flow - which don't have an Off-Peak 1st Day Return.
Really I would suggest ScotRail should roll out their GLC/GLQ to EDB fares across the Q558 to Q553 flow, but perhaps they have specific reasons for not wanting to offer the period returns and Off-Peak 1st Day Return more widely. And they should make sure the Railcard conditions are honoured by enabling all relevant Railcard discounts for all of their ticket types.
Either way, it's down to ScotRail to fix.