Could be down to what fare bucket your original PE tickets were in and if you needed to move into a S or W bucket.

As you were already in PE there would be no additional APD ( a proper tax) to pay so it would be down to a mix of change fee, VS fuel surcharge and fare bucket difference. Could also factor in if original ticket was bought in a sale.

Without seeing the detailed breakdowns it's hard to work out exactly where the differences arise.