Y07,
It sounds to me like the transition circuit is still contributing to the overall AFR under those conditions. As you correctly observed, the AFR goes richer because of the high engine vacuum caused by letting off the gas. This pulls extra fuel out of the transition slots which makes the AFR richer.
One way of limiting it is to install restrictors in the transition circuit. But don't expect miracles, because the restictors can only do so much before they limit the fuel flow to the point of causing a bog. Another method is to find a baseplate with smaller transition slots.
Honestly, an AFR of 13:1 doesn't sound so bad to me. Many Holley carbs have AFRs in the 11-12's during deceleration.