Not sure how alliance raid queue works, but I have few speculations based on my experience with other queues. There are times I've had to wait a bit as a max level AiN. Sometimes, this is just because there aren't enough ppl, ex: queue as tank for main scenario and wait a couple mins for another tank.

It also seems like there can be a delay before the needed role changes. I'll queue as tank when a roulette says AiN tank, but occasionally the queue isn't instant. I'll then switch classes to healer and get an instant queue. I suppose it's possible DPS were no longer in need when you queued, but the AiN bonus hadn't updated.