I'm glad it's not just me that sometimes gets caught out like that. My job is a big-picture scenario. I don't always know (or particularly care) if product X includes widget Y in its features, because that is minutiae. As long as product X fulfils role Z, then feature Y can generally be added if needed, or dropped if required to meet a particular deployment scenario. I don't suppose the AM cares whether there are 73, 74 or 75 in total. And even if the answer that was staffed for him was 73 yesterday, it might be 75 today. Or 69.
DCDS probably doesn't know the exact numbers of TLAMs on board HMS Astute vs HMS Ambush either. And nor should he, necessarily.
Yes, he could have thought to himself "what are these knobbers likely to ask me?" and prepared for that. But it's a bit pointless. "So, AM, of these (say) 74 combat jets, how many have had engine changes in the last month?" or "Having these aircraft is all well and good, and I'm sure we'd all agree that more would be better, what about armament for them? How many bombs have we got in store? And how many stores are there? When were they last inspected? What's the shelf life of a bomb? What contingency plans are there to acquire more bombs in short order if necessary?". Then they could get parochial. "My constituency has the Dunlop rubber plant, and it employs 1,000 people. It relies on MOD orders for tyres for these aircraft. Why have we recently switched to Michelin?"
"Sorry, I haven't a clue, but I'd guess that the Michelin one met the spec for less money, however, a Wg Cdr 4 or 5 ranks below me would have made the decision. I don't know who that is, and I certainly have no inclination to find out " is probably the answer, but that's not why military officers appear in front of HoC committees.
I feel for the AM!