I think with some agencies, the above/below phenomenon happens because the screener is rejecting manuscripts that would NEVER be a good fit for the agency (ie, your motorcycle maintenance manual isn't going to work with an expert in French history) but yours checked enough boxes that the screener wants the agent to take a look at it.
Sometimes the agent herself has screened a query into the "maybe" pile and will get to it when she gets to it. eg, your manuscript sounds interesting but she just made an offer on a similar manuscript and doesn't think she can place two such books.
The ones that bothered me most were the ones who send rejections about twenty-five percent of the time, and the other three quarters were CNR. Like really, why are you doing that? Be consistent. Why are some people special enough to get rejections and the rest of us get tossed to the back of the closet?
Don't get me started on the agents who ghost you on a full or a partial. :-b