In addition to the above, they are priced by two different TOCs.
Frodsham - Lancaster is priced by Virgin West Coast, and Helsby - Lancaster is priced by Arriva Trains Wales.
Someone with access to historical fares data might be able to explain in more details as to what happened, however the Helsby Off-Peak Return fare you saw has no time restrictions, therefore the compatible Frodsham fare I suspect would be the Anytime Return, which costs £38. Of course the Frodsham Off-Peak Return only has a nominal restriction so acts just like an Anytime Return in most cases, however that is not the point. My guess is that the Helsby Off-Peak Return and the Frodsham Anytime Return were actually the same fare types prior to "simplification". They have simply gone in different directions since then due to differing policies at Virgin and ATW.