I'd have a punt at it being because the grains get pushed closer together as more grains/rings are added over the years. The timber between each grain is compressed making for denser willow. This really spoils things for people looking for a cleft with 20 twenty straight grains that looks like it weighs 4lbs but only weighs 2'10"!
Funny, the more I read about all the timber tech associated with this, the more I'm convinced it has more to do with how good the batter is at batting than anything else. I'd heard that Albert Trott story before (they bang on about him on TMS a lot as he was the current Trotts great great grand father or something) and it supports this theory somewhat!