The concept was NOT to put a pound of lead into a shotgun at one time (EGAD! Imagine the recoil!)
As stated, a round sold lead ball that fitted the bore of a 12 weighed 1/12th of a pound. of a 16, 1/16th, of the 20 g, 1/20th of a lb. etc ad infinitum EXCEPT the .410 and the 9mm shotgun. The .410 began life based on the .44 rifle cartridge, then the 44 Extra Long, and thus never earned a gauge designation. And the gent up there is correct- .410 would be between 68 and 69 gauge. The 9mm rimfire was of Euro origin, and simply began as 9mm.
MOST shotgun gauges are in even numbers- 2, 4, 8, 10, 12, etc. The one exception that I am aware of was a VERY rare 15 g. made for a very brief period.
What we have here is... failure- to communicate.