Literary agents employ a few different rejection letters, all to suit the need of the moment.
A standard issue rejection comes in handy when the agent is swamped and can't consider another author. Something kind, of course, to let the writer down gently, but it's all quite boiler-plate. The writing might be dreadful, the plot trite, or the query nearly incomprehensible, but it's one size fits all when it comes to a straightforward rejection. Some agents will fire off something that implies they'd not mind if the author sent something else, or on the rare occasion, might suggest that the novelist query an agent friend who might like that sort of story.
Then there's the wrong form altogether. Robin Straus's assistant lifted a sheet from some other pile of form rejection letters, and I've got a rather odd reply to my query.
"Thanks for sending the material we requested," she begins. What material might that be? I only sent a query letter, and you didn't even ask for it.
"We gave this a considered reading," the note goes on, followed by the usual bit about not falling in love, etc. It was nothing more than a one page query letter, and a considered reading couldn't have taken up too much time I hope.
No matter. It's another rejection just the same, to a query letter I've stopped using because it wasn't working. On to the next agent.