As I understand it, this is the 'new' economy seat that will introduced across the range through new purchases and refurbishment of old airframes.
The 'great idea' was to make the seat a teen weeny bit wider (about 3/4 of an inch, I think) My guess is this is because the boys and girls of BA marketing realised that web sites were now comparing seat widths and they looked bad.
However, in order to give the extra width to the pax - but without increasing the overall dimension of the seat - they had the brilliant idea of making the arm rest only retract part way. This is because the wider seat does not permit the arm rest to go up.
Ta-Da! Another triumph of being able to advertise 'wider seats' but not mentioning that if you are two people (or a family) in a bank of 3 or 4 seats - you can no longer do what pax have doing for decades.
I sit to be corrected on my guess'. (And my desk chair is more comfortable than the Y chair!)