Anecdotally, they are more likely to weigh hand luggage out of the US. Other factors also affect this, such as how full the flight is.
The published allowance is pretty clear, so to take a 10kg rollaboard + backpack seems a little cheeky.

I’ve never really understood some people’s insistence (not directed at any of the comments in this thread, just a general observation) that you are entitled to a ‘small personal item’ either, as such language is absent from the EK baggage info (in contrast say, to the allowance for business where it’s clear that 2 pieces are allowed).