For example, I was messing around with one today with my own resumé. It couldn't tell from responsibilities and results that I had done a job much more senior to the job title (long and probably common story). It wanted exact matching in job title to the job description, so "department lead" and "head of department" or neither, with the same responsibilities, were not interchangeable.

A human could pick that out from the context. So I think an ATS is going to be using the filters humans used to, plus failing to gather relevant info from context.