I saw a post some time back, can't remember when or who.
It said something along the lines that certain belt clusters had a core and that they were persistent, they made a note and every so often they just flew around and cracked these cores.
I have just for a laugh tried a few and yes one or two had cores, and asteroid rocks have more chunks than ring rocks, but as I wander all over the place and the total output from each cluster is not that great, I did not think that belt mining was a viable option compared with rings.
For those that mine back in civilisation and like to pop a few NPC's every other rock, this may be for them a fun way of mining