ISTR the were introduced at the same time as the “thin-back” seats, and had less bulk and offered marginally more space.
I could be wrong, though.
The cynic in me suggests the “12” would be cheaper to produce, as being the main reason?
I covered nearly 40K miles in my XKR Dynamic fitted with the 14-way and were great. The 12-way seats do not feel any less comfortable although they miss out on the lumbar inflation.