Really for ostensible reason - cabin baggage
I have always felt that if the cabin baggage weight limit (which I have only ever seen BA and EZY try to enforce) was actually for the ostensible reason, then:
1) It would be the same in all classes.
2) It would not apply to bags that clearly fit under the seat in front.
A pilot case with just a notebook PC in it is very close to the limit, but it's still practically empty.
Momo