The problem for anyone using the west coast main line is that Avanti cap the number of off-peak tickets sold online (even though, as you say, off peak tickets can’t sell out). At busy times they’re often marked as sold out on their website and app which (they hope) will force people to buy standard premium or first class tickets instead. The way round it is to buy an off-peak on the day at the station, but Avanti don’t exactly publicise that fact so lots of people aren’t aware.
They’re an absolutely rotten company. Roll on the end of their contract and nationalisation