Thank you both.
In fact, Olivier, it was a big muffin, so I believe the OPO - FRA route was downgraded to level 4. Still, it's quite less than the small tray I was used to (2 breads, butter, jam, orange juice, a cold dish). And the flight time points to a level 5 catering, I believe (and also the purser, she told me that it should be an hot meal).
Honestly, I have no problem in paying 20€ more in the ticket to have a better catering, and, for what I've heard in the plain, some travellers do think the same.