You forget that checking a
TOC▸ website for a fare only means that the fare is available at time of checking and/or purchasing.
Same for any retail website - the cost may change at any time.
Tickets also have a validity - as does any ticket to travel. If you don't travel in the validity period, it is null & void. Thus, you need to specify (at least) the outward date at time of purchase, from which the validity period of the outward would start. Some tickets have to be started on the date of outward travel, some (singles?) are valid for 3 days including the specified date.
All this covers most/all of what you are discussing. It's the same for any travel tickets.
I do suspect that the ticket had added validity while the disruption on the SW main line was ongoing, and that got removed between the time your daughter looked it up & the time she went to buy the ticket/checked it a second time. Again, similar things happen across travel.
Had I not double checked her designated train (just to see if it was still running) my daughter could well have ended up putting forward a seemingly “the dog ate my homework” excuse - when a hungry dog actually did………………and still have got a large bill !!
She wouldn't, because she wouldn't have been able to purchase the 'wrong' ticket as it was no longer available for sale at the time she would have been buying it.