If you use national journey planner, and put in crowthorne to Reading - the 17:38 and the 17:48 are both options. An anytime single is 4.90 route - any permitted. So you would think, thats available on both trains.
WRONG.
The 17:48 you have to go to Farnborough and double back on yourself - this is given by the journey planner - but for that train it seems its 8.20 route - any permitted.
How can the 4.90 ticket be an "any permitted" when it clearly isnt - the Farnborough change has to be a permitted one otherwise why does the journey planner return it.
"ANY PERMITTED" is exactly what it says. It's not "ANY POSSIBLE".
Before the wonderfully arcane National Routeing Guide was drawn up the designation was "ANY REASONABLE". However this was seen as ambiguous, as one person's idea of reasonable may differ from another's.
In your case, you fall foul of the 'no doubling back' rule (there are, however a few easements to this rule).
Journey Planners can and do give options that appear to go against the NRG*, but unless an easement is in place you will always have to pay for multiple tickets. As to why
NRES▸ occasionally gives these strange journey options, who knows! (I suspect poorly written software).
*This was also the case when there was an online program of the NRG available. It occasionally contradicted the paper version.
ATOC» now only provide public access to a 'paper' version in the form of .pdf files. I'm not sure whether ticket offices are still obliged to hold a hard copy (or even if one is still published) for public perusal.
You can find the National Routeing Guide
here. Good luck if you attempt to use it. Fiendish, to say the least.....