They are exactly the same material ie wood fibre BUT the wood pellets are simple compressed sawdust which is already in such small size particles that it rots down much quicker. The shavings being much larger and much more fibrous have a lot more work to do to turn themselves into black gold, and getting that reaction going in those circumstances means initially they can suck goodness and nutrients in rather than giving it out. Also in our case the hemp is mixed in with the wood pellets and the hemp is reknowned for rotting down mega quick so it helps to get the process going too.
We use the hemp/pellets mix as unlike shavings it doesnt stick to rugs, horses and fleece jackets, and doesnt blow everywhere and the horses love it, it's like a lovely forest floor! And the mucky bits stay where they are and dont get dragged round the stable. Also shavings bales often take ages to fluff out as they can be quite compressed, whereas pellets and hemp both just pour out :-)))) yes I am a bit lazy!