Why does the score increase as the square window angle actually decreases?

Although it seems counterintuitive, the square window which measures the angle to which the wrist can be flexed passively, does indeed DECREASE as maturation progresses.

This is so because Passive Flexor tone increases progressively during fetal maturation. The entire neuromuscular portion of the assessment is based upon this fact. At term and post term, the infant has maximum passive Flexor tone and minimum passive Extensor tone.

AFTER term and post term, this maximum PASSIVE FLEXOR TONE DIMINISHES over the first year of life until there is a balance between passive flexor and extensor tone.

Dr. Ballard

References :
Ballard JL, Khoury JC, Wedig K, et al: New Ballard Score, expanded to include extremely premature
infants. J Pediatrics 1991; 119:417-423.