At the operator's website (in this case www.northernrailway.co.uk)you can still buy an Off-peak Day Return for today for £23.10 - though you won't get much sightseeing in now! This should also have been on offer at the station booking office.
Maybe you left Royston during peak hours? Or maybe you didn't ASK for a cheap day return? I'm afraid they don't always thrust the cheapest option into your face.
SNP and Greens end coalition deal
Scottish political mess. Is Devolution working?