The usual ways to determine your PAP involves looking at the first oil rings on the ball. If you do not set the ball immediately down on the lane, if it rotating in the air at all, it can flare off the initial axis of rotation before it makes contact and the first oil ring is not centered around the initial axis of rotation.
On a lower-flaring ball, the axis of rotation will not change significantly while the ball is in the air (or on the lane, for that matter). Spare balls tend to flare very, very little and so your PAP measurement will be extremely close to the true PAP. With a high-flaring ball, the axis of rotation can migrate enough for the error in the PAP measurement to be significant.
SH