Chapter 5-6

Percy is placed in Cabin Eleven because he is considered undetermined. Hermes is the god of travelers, thus, he looks out for the campers until they're determined.

“You’re undetermined,” Luke explained patiently. “They don’t know what cabin to put you in, so you’re here. Cabin eleven takes all newcomers, all visitors. Naturally, we would. Hermes, our patron, is the god of travelers.”