What doe Slim find ''funny'' about George and Lennie's relationship?

Slim finds it odd that the two men are traveling together, which in itself is odd. He finds George's situation even stranger when he learns George and Lennie aren't even related.

George explains that he was friends of the family, and Lennie's, Aunt Clara asked him the care for Lennie after her death. George complies with her wished, and actually, he enjoys Lennie's company (in between the trouble).