Going off of Table 1-2 in the Bestiary isn't really enough. You also need to cross reference Table 1-1.
Looking at Table 1-1 it's pretty clear that for the most part the Hit Points, AC, Ability DC, and saves are closer to the CR 5 stat line. Attack Bonus is higher than average but average damage is lower than average. The result is that on average the Green Hag aligns closer to the CR 5 baseline than the CR 7 baseline.
It's also helpful to look at the other monsters at a given CR level in comparison. The Medusa is a CR 7 Monstrous Humanoid and is a far tougher threat than the green hag IMHO.
just ran a hag/witch centric story arc, and the Greenies are wierd liek that. They are really only dangerous en masse, as part of a coven, OR when you use them as the spy/assasin they are stated out to be. In a straight up fight, they are weakish, but if they have the support of the town guard because the Captain is her willing love slave, well....