I do not know why some would take longer to slow down then others, but I do know the reason for the spin-up. With the old quick change chuck, if you didn't get the bit seated properly, the spin-up would make the bit fall out rather than going into the wood at full speed and shooting it out.
I was also told that everyone should make sure their software is up-to-date and not using older versions.
Also, I sent a link to this thread to an engineer to look at.
From what I understand, all B & C machines had the metal clip on the encoder.