The omitted columns are #11 and #12. On FMP's Original Forms page, the Enumerator's Instructions sheet states:
Column 11 - The enumerator may be instructed not to transcribe the schedule entries as to "Membership of Naval, Military or Airforce Reserves, etc., or Civil Defence Services, etc." from column G of the schedule [...]. But if he is instructed to transcribe those entries, the entry should be made in column 11 on the right-hand page. The entry, if directed to be made, should be an exact copy of the entry in the schedule.
The schedule form (as in your question here) shows that column G is indeed for membership of military or civil defence services. It's not clear why that information was only supposed to be transcribed on demand, though. It's certainly a shame that it's missing from the images.
This example of column 11 (from this record) shows three entries, for army, air raid warden and mercantile marine, which is consistent with the note above:
The twelfth column is labelled "Postings", and so was presumably intended for tracking the whereabouts of military members noted in column 11.
I've seen other notes on the right side of the page that don't appear to be related to military service,and are clearly not in the enumerator's hand. I suspect that once the register passed to the NHS, that side of the page became a general notes field, much as described in @JanMurphy's answer.