Another possibility is that the measured neutral current is reduced because some of the neutral current is flowing through the EGC or other ground path.
This is happening because:
1. there's a connection or fault between the neutral conductor and ground further down toward the loads,
or
2. one or more of the loads is connected to the EGC instead of neutral
This is something I've been wondering about....
This circuit, like I said, is a string of street lights in our small town. The wire that daisy-chains from light to light is direct burial and was probably installed before 1980 (just a guess). The fixtures are definitely from the early 80's. The circuit is on a 2-pole 50 amp breaker. These lights have been serviced as needed by the town maintenance guy (i.e. not an electrician).
Periodically this circuit trips. It has happened more frequently this fall. I'd say it might've happened 4 times since August. We keep looking for a pattern related to occurrence such as ground moisture, etc. but there doesn't seem to be a common denominator. It happened again Thursday night. Previously the last trip was around the first half of October.
Yesterday, based off a previous suggestion by Larry Fine, I installed a different breaker that has the handle-tie removed (it didn't say "common internal trip"). I'm hoping that whenever it next trips I'll be able to see which leg trips to help narrow down the issue.
The town is at "year end money", so they don't have money to throw at this issue. I'm hoping if I can identify the issue I can inform the city council and they can work a fix into the budget for year 2020.