I cannot comment on their current practice, but they used to have the books assembled overseas, and bound in the uk.
Reasons were twofold: 1. The spine doesnt melt in a hot container like Battlefronts books often suffer from. 2. It avoided some sort of import tax due to not being a finished product.
I have absoloutely no idea if this is still the norm, I'm pretty sure they moved the contract away from the company who used to do it.