Not sure which day OP was looking at, but seatmaps in ExpertFlyer show for AC001 tomorrow:
- 5 of 20 seats unassigned for J (25%)
- 16 of 20 seats unassigned for PE (80%!)
- approx 70 of 210 seats unassigned in E (33%)

Of course, that's just one day, and we know unassigned seats is not the same as unsold capacity. But it does indicate AC hasn't yet become terribly effective at selling PE. Part of this may just be consumer unfamiliarity -- but pricing them low for now may be just sound supply/demand, and as other have said, if it muddies the waters and even occasionally underpricing E may benefit AC in the long run.

To add the the muddiness, it feels like PE Lowest competes with both Tango and Flex, so the really surprising bit would be if ever PE < Tango, not necessarily PE < Flex. I know that feels absurd to those of us here, who pay attention to mileage accumulation, upgradability, and seat selection, but that doesn't describe a lot of the flying public.