Sam's yo-yoing artistic ability

Either it's just me or a little canon discrepancy. In “Home” Sam draws a beautiful picture of a tree he’s seen in his prophetic dream that clearly shows his artistic ability (it’s necessary for a hunter to be able to draw – how else would they paint all these intricate protective and demon or angel-invoking sigils?). But later, in “Bedtime Stories”, when he’s doing an identikit of a murderer from a description by his surviving victim, he comes up with a child-like sketch bearing little resemblance to reality. So, maybe Sam draws well from memory and imagination but not from verbal clues? Or maybe he’s better at sketching inanimate objects?