The reason for that is that the MB is almost in the track. I remember reading that the reason you get the same amount of flare from asymmetrics regardless of the how far past 3 3/8" the pin is from PAP is because of the migration of the mass bias. After the pin has flared, the MB starts to flare as well. Putting the MB in or very close to the track takes it out of the equation, and you end up drilling like a symmetric ball.
SH