[shasta]

Right, but "recite" was editorializing. That implies that the interviewer expected you to produce the answer from memory, as opposed to thinking about it. It's an easy question if you're familiar with moving window averages and know what the interviewer intends. If it was asked apropos of nothing, a request for context seems reasonable, though. It sounds like you probably had a bad interviewer. There seems to be no shortage of software interviewers lacking in "people skills."