Sometimes asking "why" can be seen as confrontational. If they asked you for X and X was not mentioned in the instructions or the report (a lot of stuff might be in the report but not in the instructions; we are required to obtain the information needed for the report), either you are able to give them X, in which case you should simply give them X, even if it was not in the instructions, or you are not able to give them X, in which case you should tell them, "The report did not ask for X and I did not get/notice/photograph X because I had no way of knowing you might need it."
But asking them "Why are you asking for X now? It wasn't in the instructions" isn't going to make any friends. Quite likely the reason they are asking for X is because the client requires X and they may have screwed up by not noting X in the instructions. So they're trying to give the client what the client requires, hoping maybe you did get X even though it wasn't asked for originally.
(Or they did ask for X, you overlooked that requirement, in which case you should have apologized and offered to go get X now. In that case it doesn't matter why they need it; you should have gotten it.)
Ours is not to question why, and if that's what you did, that's why they got upset.
Time to build a bigger bridge.