I'd argue that the effect of cannabis on creativity is caused, in part, by an increased appreciation for "the little things".

Before the high sets in, one of the first things I notice is an increased appreciation for the thoughts that I have. I find myself proud of having certain thoughts (jokes, ideas), instead of feeling like I haven't done enough.

So the high helps reinforce underappreciated trains of thought. This helps boost creativity because all those little neural pathways that used to lead to nowhere, they become blissfully incorporated into the high-creative mindset.