Balance is reported as a negative number when the sum of all 50A circuit channels is greater than the number reported by the Mains. The most common reason for this is a multiplier on the dual pole circuits. While some 3 phase circuits have a balanced load, they can also be different. This causes the multiplied value reported to be greater than the total actual load.
For example, the heating element of an electric dryer might be on one leg and the tumbler motor could be on the other. If you are monitoring the tumbler leg and the dryer heating element goes off, you might see an overstated value due to the multiplier.
Also, if you are not using the 200A sensors to monitor your mains, but rather an individual circuit, you will likely see an Balance value that is negative.