This has been shown dozens of times to be an unintentional artefact of how slow DF updates and how the system was when AIN was released

Tank is “first priority” for AIN which people believe is a reflection of the fact that when it launched tank was the rarest role. Healer can only be AIN specifically when tank is NOT AIN. If both are needed then tank is given AIN even if there are actually less healers

This leads to the situation where you are a tank and queue for something as AIN and the queue takes like 5 minutes but if you leave and queue as a healer you’ll get an insta pop, both roles are technically needed but even though healer is “more” needed tank is the default AIN. You also notice this where if you check AIN at the dead of the servers downtime when nobody is queuing or right after downtime tank is just the default AIN