Salix do know their stuff, and if we are saying that G1 clefts should be only all sapwood, then what they say makes sense. However, we know this isn't the case, as the grading standard that each company uses is different, and quite often inconsistent.
Trees get narrower as you go up, therefore the grains are tighter, but you also get more heartwood. (G2) Also, you would assume that there is more water retained lower down the trunk due to gravity, which supports the theory that there is more moisture in the wood.
Was interesting, I remember SAFbats saying that more grains means the wood is more dense, however, Salix and Gray Nicolls both say that grainier clefts tend to be lighter. That threw a big spanner in the works, as I used to follow Norbs' blog, which was interesting to read, but I know who I would trust more when it came down to the properties of willow.