I'm mostly an AA flyer domestically, but I'll give you the following:
Sometimes, I'll encounter fares that are really close, Y vs J/F. Usually it's either a fare booked close to flight time or a short hop flight (CMH/ORD or CMH/DCA).
Like you've encountered, sometimes the prevailing premium fare is less than the Y fare.
I've noticed a lesson from this forum.... Usually it's a Y fare with few or no restrictions versus a restricted first class fare (for example, no refunds available on the F fare, or particular days/routes restrictions).
Hope that helps.