Sorry for the brain fart. The timing is set at 20dBTDC and actually occurs at about 16dBTDC, obviously not at 16dATDC! I guess I got my As and Bs mixed up!!!
This probably happens as a result of the timing gear lash and delay, but there could be other reasons in addition. This does not happen with the two single mags. We have been watching this on the test stand for a long, long time.