VWC-priced 'Any Permitted' SVR (Off Peak Return) is £66.10, restriction code 9I, is a M-F restriction, allows departure from EUS 0905-1514 (incl.), and at or after 1832, arrival at or after 1055. It allows departure from STP 0901-1629 (incl.), and at or after 1859, arrival at or after 1006.
EMT-priced 'Rte Chesterfield' SVR (Off Peak Return) is £85.00, restriction code 9J, which is a M-F restriction, allows departure from STP between 0901 and 1629 (incl.) and at or after 1859, or outward portions from STP are valid anytime on Fridays after 0901. Arrival restriction is at or after 1006.
EMT-priced 'Rte Chesterfield' SSR (Super Off Peak Return) is £64.80, restriction code CJ, which is a northbound M-F restriction, allowing departure from STP between 1026 and 1515 (inclusive) and arrival at or after 1129.
From that information, I think you are correct! How unfair, as well as confusing, for the passenger! Who would pay more to use a slower route?