Well, normally, if eating out-house, one does so in a restaurant - and there the customer is King... left vs right? Who cares, as long as the customer pays?
But an official banquet is tricky. Normally the man is seated left from the woman and if he helps her to sit down with her "complex" evening-gown, then she has to enter the chair logically from the right, since the man is standing left, manipulating the chair.
So, the Armed Forces... Mainly only men I assume and the women not in evening-gowns? No need to help... what could be an explanation...
Disclaimer: I am from Germany and have absolutely no idea about Commonwealth Armed Forces etiquette!