The AHL is a lower skill league, obvious since the NHL is the big show, but as such, developing prospects and those not good enough for the NHL play there. There tends to be more head hunters and cheap shot artists in the AHL simply because they aren't good enough for the NHL and I guess they need to do whatever to keep their jobs.
There are skilled players in the A of course, but the best and the most developed are in the NHL. Therefore, for higher end prospects, or more talented players (who really need seasoning to reach their ceiling), they have less talent to play with in the AHL. Using Caufield as a potential, he might have trouble scoring not because he isn't good enough to beat AHL goaltending, but because the players around him don't get the puck to him. On the flip side, this may help him develop the ability to generate his own offense, but he'd probably have an easier time scoring in the NHL, even though goaltending and defensive systems are better.