If same price, might as well just sell returns, and either use or throw away return portion as per your needs. This happened to me a few years ago at Glasgow Central, I asked for a single to Exhibition Centre and was given a return, as it was all there was/the same price.
If you ask a person for a single and a return costs less, then sure, overwhelmingly they will suggest a return.
However, the Hull Trains app and website aren't people, and if they're asked for a single they will not show you or warn you that a return may be cheaper, so unfortunately in this particular case I don't think your point stands. It would make things
so much easier if there were some logic in place to detect all cases where a flexible return type may be cheaper than a flexible single type for the appropriate journey, and add a single of the comparable type at either the same or 10p less. Some TOCs are very good at ensuring this when they make changes, and some aren't.
For example, at least four years ago both me and someone internal at LNER sent messages politely pointing out that there is an Off Peak Day Return from Wakefield to Doncaster at (in today's prices) £9.40, but no comparable single with the cheapest one costing £11.10. Unfortunately, no Off Peak Day Single was ever created, despite that this would make life easier and simpler for the staff, would be more consistent with other local journeys and other short LNER journeys, and would mean the website and app wouldn't overcharge. LNER have however since added their own Advance tickets between Wakefield and Doncaster, along with just about any journey you could possibly make by LNER, even Haymarket to Edinburgh.