Approximately 8 years ago I built (with the help of a contractor) a
1,200 sq. ft. shop detached from our home (nice shop with central heat
and air, half bath, hot water, etc.).

Our home has a 200 amp box so I simply put in a 100 amp breaker and run
it to a sub panel in the new shop which has a 100 amp breaker box.
Everything in both the shop and house has worked fine since the
beginning and still does but I did find something somewhat unique.

I have one item in the shop that draws 23+ amps on a 115 volt circuit
and to my surprise had never popped a 20 amp circuit breaker. All
outlets and lights are wired with #12 wire so I know that 23+ amps is a
bit too much but so be it. Well, I moved the item a small amount and
happened to plug it into a different outlet which happened to be on a
different breaker and it popped the breaker after running for about 20
minutes. Tried it again and the same thing happened. Out of curiosity I
checked the voltage and that particular circuit (the one that the
breaker pops on) has 125 volts and all of the others run in the 115/117
range.

Anyone have a clue of how this could happen when they are all coming out
of the same box and are approximately the same distance from the box?

BTW - if you intended reply contains the word "code" or "insurance"
please don't post as it has nothing to do with my question.

Call your power company and have them check the service drop to your house. A bad connection could cause some voltage drop.