It is booked as a 365. and most diagram do run as booked.
ALL diagrams are interchangeable.
i.e. 05:51 SRS-BDM is booked as 2x 319 but for the past two weeks it have been 2x 377.
Wouldn't entirely agree that everything is interchangeable on the GN side. None of the three classes, 317 321 and 365 can work together in normal service (though I'm not sure why 317s and 321s couldn't work together in service with little or no modification if required, whatever the reason it's never happened since the 321s arrived).
For starters, you have three morning peak stopping trains from Cambridge to King's Cross which *must* be formed of 2x317, as these trains are the only ones fitted with door deselect for stopping at Foxton, Shepreth and Meldreth. If 2x317s cannot be provided, the train must non-stop these stations, or be formed of a 4-car train only.
Then you have many diagrams which join other diagrams at various times in the day. Splitting and joining is very common particularly at King's Cross and Cambridge. Indeed, as many as most Kings Lynn services nowadays detache or attach 1 or 2 units at Cambridge. You can't just sub a 321 in place of a 365 without the wider implications being considered - if a 321 turns up at King's Cross on a 365 diagram timetabled to join another 365, there will be a problem, and some form of adjustment will be required.
Finally, there seems to be a policy of keeping the Kings Lynn services worked by 365s as far as possible. There are no 317s and 321s booked to run beyond Cambridge, although they regularly turn up on substitutions as far as Ely. Generally 317s only reach Kings Lynn in emergency. I'm not even sure if Kings Lynn drivers sign 317s or (more so) 321s?
Regarding the OP's question, there is currently a long-term shortage of 365s due to the refresh programme. The 365 fleet is tightly diagrammed even with all 40 units available, the 317s and 321s are less intensively used, with 2x317s and 1x321 booked spare, and many diagrams involving little work. The entire 321 fleet stables between the peaks. There are a number of diagrams which are "favourite" for subs because they don't divide and join (avoiding the complications described above), and in some cases start and finish at Hornsey.
I haven't got the diagrams to hand, but it seems your service is one of the diagrams which is ripe for subbing if required. Normally it seems to rotate around so that different diagrams get subbed each day, but you will find that if you use one of those diagrams then it's likely you will get 317s or 321s probably at least one a week for the foreseeable future. Things will improve once the 365 refresh is complete, of if the timetable/diagrams change so that your service becomes a diagram which can't easily be subbed.
I should add that the subbing can be quite complex. For example, if they are one 365 short and have only one available spare 317 (so can't make an 8-car), they might choose to put 1x365 on the all-day 1x321 diagram, releasing 1x321 which along with the spare 321 can be subbed for 2x365. Things really are that tight.
FCC made moves to hire 2x317/7 precisely for the reason that they knew they were going to be down on 365s, however for some reason it was decided that they could handle things without. Generally this has proven to be correct, although it does leave them vulnerable if more than one or two units are unavailable for any reason.