No, that's not the reason.

The seats the OP has sat on (I suppose on the 787-8 or -9) were made by Recaro and launched in the late noughties. They feature the 'hammock'-like headrest and they were specifically designed to have a slightly larger seatback - Recaro said it was for comfort - and as a result the space between each seatback got smaller as the seat went, shall we say, 'up'. You can see it here on G-RAES when it was being delivered after its first refurb.



Now, instead, the standard is this other seat, I think the CL38something, still from Recaro, which is straight in its elevation, thus giving space for the armrests to go fully up. I'm posting a photo from an IB 350, but it's the same on BA's 78X, 350s, retrofitted 777s as well as other airlines like JAL on the 350.


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