I think it's like that (correct me if I'm wrong): Before the apocalypse, your guy was a carpenter in his "main job", and have the gardener trait - he probably did it like a hobbie - that would explain why he level up farming faster than any other unrelated skill, like aim and first aid. It makes sense, because it's harder to learn a skill you don't have any practice than one you had as a hobbie (gardening in that case). And I have no idea how books will work after the update :/ Sorry if I said wrong things, but that's what I could get lol (also sorry for my bad english please hehe)