Apparently, as far as I can decipher this book, it seems that after your human PC hits 21 POW via experience checks, you can't raise it this way further. No minimum chance or anything, other than the vague words on the paragraph right above that part, where the implication is that POW is the only attribute that's supposed to be NOT limited by the species maximum.
Was this part incomplete, somehow? Are the only other rule codified ways to raise POW above 21 is to gain or start with a POW increase Mutation, the Sorcery spell "Bolster the Soul" cast on max level, or Super POW?