Actually, Allvest did show both the Premium Economy lowest and flexible in his post.

I have seen Premium Flexible both slightly higher or slightly lower than Latitude.

There is an opportunity now for AC to encourage fliers to try Premium Economy, albeit limited right now due to the number of aircraft with that cabin. Obviously that will change as the 789s arrive.

In addition, there were some specific promotions that offered bonus miles or a discount on certain routes, another incentive to try it.

So yes, IMO, there is often much better value (at least right now) in the Premium cabin on some routes on some dates.