I also thought I read that versions of romsets werent necessarily recompiles of every rom, but were just the new roms that were added as of the updated paired version of Mame itself - i.e.
Then run a scan job, with all the fix options ticked.
Bad dumps may have been fixed, roms may have been moved to more logical places (into or out of a driver/bios/parent/child sets), new drivers may be added, some roms may be renamed.
The cumulative effect of these small changes over a large number of versions can pretty big.
Do I actually need each one, or is there a better way?
Or should I simply look into a different rom manager instead of clrmamepro? Given the distance between your existing romset and the current one (138-171) I'd just download a new complete set.