There's probably some discrepancy because at any given instant there may be two or more people queuing or dropping out of the queue. Since they most likely don't update the interface in real time, I wouldn't be surprised that you get no healers when it says it needs tanks, because some other tank had joined just before you and snatched up the last healer.

There's also the case when two roles are missing people simultaneously, then most likely it will ask for tank first simply because it looked at the tank queue first.