I plan to book an off peak day return ticket (CDR) this weekend from Southampton Central (SOU) to Yarmouth IoW (YMH) with a 26-30 railcard. This journey involves changing trains at Brockenhurst and then switch to Wightlink ferry at Lymington Pier. While booking the ticket, I was confused by the fare system.
It seems like the discount rate for this journey is not as much as 34%. More specifically, a full fare SOU-YMH CDR is £28, thus 28 * 66% = 18.48, while a 26-30 railcard discounted fare SOU-YMH CDR is £21.10. However, other railcards, such as 16-25, Network and two-together, do not have such problem and all have a correct fare of £18.65.
I searched for more journeys and found that the issue is in CDR and SDR fares to YMH priced by SWR, GTR and Chiltern, while other tickets, such as anytime, super off peak (evening, Sunday) and all Single ones, as well as fares priced by GWR and XC, do not have this problem, examples given below:

Could anyone kindly help shed some light here please?
It seems like the discount rate for this journey is not as much as 34%. More specifically, a full fare SOU-YMH CDR is £28, thus 28 * 66% = 18.48, while a 26-30 railcard discounted fare SOU-YMH CDR is £21.10. However, other railcards, such as 16-25, Network and two-together, do not have such problem and all have a correct fare of £18.65.
I searched for more journeys and found that the issue is in CDR and SDR fares to YMH priced by SWR, GTR and Chiltern, while other tickets, such as anytime, super off peak (evening, Sunday) and all Single ones, as well as fares priced by GWR and XC, do not have this problem, examples given below:

Could anyone kindly help shed some light here please?