The US also allows emotional support pit bulls and carry-on luggage to block the emergency exits.
Ultimately the rules are different in Europe vs. the US. In Europe, whenever a second piece - i.e. personal item - is allowed, it needs to fit under the seat in front of you. If both pieces are too large and one can go in the hold, then one will go in the hold...
https://www.iberia.com/gb/luggage/hand-luggage/