Here are a few more thoughts.
Take a sample of the water in your sump to someone and have it tested for Chlorine. Maybe your local water company will do this. The presence of chlorine would indicate treated ( municipal ) water rather than ground water.

Another possible source could be a leaking blow-off valve for the water heater, although you'd notice that lack of hot water.

House built in 1978, hmmm. Could it have the blue polybutelene ( spelling?) water line from the main to your house? Is there a patch of really green grass that always seems to grow even when the rest of the yard is dry and dead? Does that patch of green grass coincide with the water line from the street to your house?

Water meters inside and outside your house probably show different numbers, as they were probably installed at different times. What you need to do is check the water used in specific time period on each meter--looking at the difference between the two readings on each meter--that should agree.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags