Myself and my wife flew from EDI to LHR with BA, then on separate tickets from LHR to LAX using the most American of the US flag carriers, and our bag showed up as 3 kG heavier at LHR compared to the reading on the scales at EDI. We hadn't even opened the bag to add anything, so it is not just the low-cost carriers.
(also, on a different occasion, the check in person at LHR could not comprehend how it was possible to be in possession of both a valid ESTA, dating from March 2010, and a US visa, valid from about six months later
)