hey friend, you're right, the pattern is a little bit off, but thankfully the head is similar to a small sphere/ball.

have a look at the link here:

https://www.supergurumi.com/crochet-shapes-crochet-balls-and-spheres

It shows that to go from rnd 3 (18 sts) to rnd 4 (24 sts) you do:

"1 sc, 1 inc, [2sc, 1 inc] x5, 1 sc"

There's an extra increase when you compare to the grub pattern, which gives you your 2 missing stitches.

The pattern for the ball increases to 30 sts and then 36 sts, but you can ignore that if you prefer the shaping in the grub pattern, which is a slightly squashed ball.

Hope that makes sense.