I think they are, although you may have heard differently.
Humans can't help but be more attracted to some things than to others. Even if it's a person's job to read a whole bunch of things, we tend to want to read some of them and not want to read others. I know this from years of sophomore essays.
So a submission must either appeal to an agent or not upon reading the query. We know they get tons of them. Do they save the good ones for last, as a reward for their labors, or do they shove the not-so-intriguing ones to the bottom of the pile?
I suppose it's a personal decision, and it could vary with a person's mood. I recall that some nights I read the essays with the worst handwriting first, to get them over with, but other nights I started with one I knew would be well done, to sort of get me in the mood to finish them. Of course there were nights when I just read through the stack as it came. No choices, just get it done.
So there's my question for the day. How do agents decide what they will read and in what order? Unlike English teachers, they don't have to read them all, and they don't have to read all of any one submission. They have that tiny bit of luxury I never had as a teacher: writing "Not for us" in one corner and going on to the next.