As Adam said, people will Grade differently. I've seen lots recently online as G1+ which are imo G2. However, most people buying bats don't care too much, or don't understand. A bit like how not many people consider if the bat will most likely be drier, or what handles are used etc.
I will generally clefts at the grade i got them in at or grade some down. On a rare occasion one or two will go up if after planing the face and trimming the cleft to size, certain blemishes disappear. But generally people may get a very nice G3 or G2 instead, like in my recent post for example. Which many would class as a G1 imo.
My current bat I've used now for two seasons is a G2, will be using it again this year as I love it. Grading is asthetic imo and that of many other batmakers I've spoken to.
The huge demand for willow will mean the really grainy ones become more and more rare. This will drive up their price and no doubt mean grading will change slightly spark across the grades.