Why the difference in duration?

[Alberel]

SE just threw everything but the kitchen sink at AST to try to push it into viability. It's...really not a very well designed job, so they only way to make it relevant was to overtune it.

This is more or less the answer, yes. Since AST has no real niche in terms of its healing ability, and instead is just a poor hybrid of both other healers, the only way SE could make it viable was to make it OP. Why on earth they ever thought it was a good idea to combine two existing healing roles to make a new one is beyond me.