Because the BX has what is called "Limited Cat 1" capabilitly, the XB series attachments are built with Cat 1 pins but those pins are set at Cat 0 dimensions so they'll connect easier and the geometry will work better.
As just one example your hitch will not lift things as high off the ground as a normal Type 1, because the tractor is so small, so the augers for post-hole diggers must be shorter, in order to give you the necessary ground clearance. Also, many Cat 1 attachments MAY be too heavy for your tractor to use effectively, so just because it says Cat 1 doesn't mean it's a good fit for your BX...
The XB attachments WILL FIT your BX -- they're designed for it...
You can't use Cat 0 attachments, as is, because they use smaller pins, and the Cat 0 PTO runs at 2000 RPM, not 540...
It might be possible to put Cat 1 pins on a Cat 0 implement and use them, but that depends on the specific implement...