I'm sure that i have said this somewhere before.
There is no universal grading system. Some willow merchants grade differently to others. When these graded clefts arrive they are often graded again by the bat maker, and the bat maker will grade as they see fit. Other bat makers source directly and not through a merchant so just grade things themselves, everyone is using a different set of criteria. Some people take grade 1 to mean a totally flawless face with no heartwood, others allow heartwood and even very minor blemishes into their grade 1.
Basically it is all opinion because there is no standardised set of rules.
If I cut a 2 grain leg that is full of knots off my table, splice a handle into it and sell it as grade 1, it is grade 1 because I the manufacturer say it is. Whether it is a cricket bat or not is another thing but you get my point hopefully.