As I recall from the years and decades, Browning came out with the X bolt some years after my buying an A bolt.
Many of the A bolts were with the BOSS system and I notice the X bolt isn't.

Could be based on shooters that loved the BOSS and those that disliked it.
Being a more expensive rifle and me being cheap at the same time, I loved using the BOSS to come up with five different grain weights in one rifle, from 125 to 180 grs.; that shot exceptionally accurate. Took the place of at least three rifles. Fit my cheap mode perfectly.