The wait time for tank is going to be skewed compared to other content because the party makeup is different: the balance of tank/healer/DPS is 1/2/5 for alliance raids vs 2/2/4 for everything else, so tanks are in lower demand and there is less of a buildup of DPS. On my data centre at least, I regularly see DPS listed as in need for alliances.

Additionally, from my own observations, I think that the "adventurer in need" bonus can lead to an overstock of tanks in lower-level content because if it's trying to find a tank for Lv70 or 80 content but people keep queuing as tanks that are ineligible for that content, they instead pile up in the queue. And all the while, tank is still listed as in need. I could definitely see that leading to a long wait time for tanks while actually you'd get in quicker as another role, maybe even DPS.