You're not getting it, SE doesn't set the AiN, they don't have a set formula that selects tank 95%, Healers 4% and dps 1% of the time. The system itself looks at the numbers of ppl queuing as the various roles and see's which one is the lowest (based off whatever metrics they use) for each specific content.

It's not there to give some classes a boost or bonus, it's there to try and encourage more of the lacking role to join up. The only time it's going to not be tank, is when a different role is more 'In Need' because there are plenty of tanks. Since a lot less people play them, it's generally not happening.

You want the extra reward, fill the role that is currently most required.

Essentially you're saying to hell with role balance, I want a random bonus given out to various roles so I can get extra stuff for a different job.