A post from trackitt.com explaining possible reason

The moment someone enters the I485 club, it is clear that his PD is "current" as of that day, and that is why he/she was in the club. But between that time and the case coming to adjudication, if the visa number is not available (not as per PD, but just the availability of visa number from DOS), then such cases will be stacked in a separate shelf sorted by the PD. These cases will be reviewed periodically (monthly) and approved. So, I still hold the point that PD is not considered for adjudication, only visa availability from DOS at that instance of time.

--- 总结一下是说排期只是个对供与求的估计，排期到了不一定有号儿（特别是现在财年马上就要结束，中印这样僧多粥少的国家号儿已经没了），不current不一定没号（特别是那些僧少粥多的国家）。批不批关键是有没有号儿，而不是排期，特指对现在财年最后一个月，有号儿不用肯定作废，不用省着以防PD早的NC突然在这最后一分钟过了给耽误了。

我觉得少数本月批了的幸运儿们是用了海外领事馆事先申请没用完返还的号儿。如果我的估计没错儿，下个月新财年开始，批准会多起来。

个人观点，欢迎讨论。