Sometimes AC adjusts Y/J sales limits based on expected demand. That is, if Y demand is high but J demand is soft for a particular flight, they can "convert" unlikely to be sold J seats to Y seats in the reservation system, allowing them to generate at least some revenue and creating op-ups at the gate should the need arise.

If one were tracking such a flight and were to take data points from just before the conversion and just after, it would look to the naked eye that, say, 10 seats had been sold overnight, when in reality the number of confirmed J passengers was constant.

Not saying this is the case with your flight... just something to keep in mind.