Disk utility is reporting 217GB, of which 81 is purgeable. This means 136GB of actual free space and 81GB of stuff that the system can delete. (And if you add that 136GB to the 108GB used, you get 244, which is almost identical to the drive’s capacity). I think Disk Utility includes storage that can be made available by deleting Time Machine local snapshots.
Finder’s report is 211 available is almost the same. Finder doesn’t (as far as I know) know about snapshots, so it is reporting slightly less free space than Disk Utility. Does 6GB sound like a reasonable amount of space to be consumed by snapshots?
I’m guessing that the remaining purgeable storage is system caches and temporary files. There can be quite a lot of that, especially if you do a lot of web surfing. Try deleting the caches in your web browsers and see what that does to the numbers. Also, locate any locations where temporary files are stored and purge them to see how that changes.
Finally, I think purgeable content includes the contents of /var/folders. This contains a lot of temporary- and cache-line stuff. I wouldn’t recommend deleting anything from there, however. The content in there that is truly unused should get purged on a schedule and the rest is used by the system. You may be able to delete the files, but chances are that much if it will be re-created over time, and you’ll suffer a performance hit as that happens.