> humans technically can eat it, but it's chewy and not very good.

Not just technically. It is a relatively common food. A fair bit of it is crushed (i.e. turned into cooking oil). But it is also a product used in a number of processed foods, tofu, etc. Granted, it does seem to be eaten less commonly in the USA, but is more often used in Asian cuisines.

> Are there different grades of soybean?

All crops have different grades. Poor weather conditions is the most likely reason for a downgrade.