That's only because VT's franchise agreement prohibits evening peak restrictions northbound from MKC. They would impose them in a flash if they were allowed to - in fact they did for a bit and had to remove them when they were pulled up on it.
I recently came across this issue. A friend bought an Milton Keynes C to Manchester Stations Off-Peak Return. NRE confirms (scroll all the way down to restrictions) that BR Fares is not wrong and the restriction code is 9I
9I said:Outward:
Not valid on trains timed to depart:
London Euston after 04:29 and before 09:26 or after 15:00 and before 18:45, except for Easements shown below;
London Kings Cross after 02:59 and before 09:06 or after 14:59 and before 18:59 (and before 18:15 on Fridays);
London Liverpool Street after 04:29 and before 09:31 or after 16:28 and before 18:35;
London Marylebone after 06:29 and before 09:00 or after 16:00 and before 19:00;
London Paddington after 04:29 and before 08:10 or after 16:40 and before 18:31;
London St Pancras International after 04:29 and before 10:26 or after 15:15 and before 18:59;
London Waterloo after 04:29 and before 08:15;
Kensington Olympia after 04:29 and before 09:15 or after 15:14 and before 18:12;
Clapham Junction after 04:29 and before 09:30;
Bedford after 04:29 and before 09:01;
Luton or Luton Airport Parkway after 04:29 and before 09:01 or after 16:29 and before 18:59;
Milton Keynes Central after 04:29 and before 09:51;
Reading after 04:29 and before 08:35 or after 17:10 and before 19:01;
Stevenage after 02:59 and before 09:19 or after 15:06 and before 19:01 (and before 18:35 on Fridays);
Stratford (London) after 04:29 and before 09:30 or after 16:29 and before 18:34;
Watford Junction after 04:29 and before 09:38 or after 15:00 and before 18:38.
etc.
So it says one is not permitted to use a train timed to depart London Euston after 1500 and before 1845.
However if you go back to the journey planner it happily offers you the ticket for all of the trains that do precisely that. Other journey planners agree, including VT's. The button that says 'other services you can use' even shows you these services can be used.

So what gives? Have they forgotten to re-write the restriction text? Seems unlikely. Perhaps more likely is that the point at the bottom about being it not being permitted to depart Milton Keynes Central "after 04:29 and before 09:51" is intended to override the other restrictions. Interesting!