The neutral return current is confusing to a lot of licensed electricians.
When wiring a house, the installer distributes the 120 vac loads, which are light fixtures and outlet sockets to try and balance out the possible 120vac loads on each half of the 'transformer'. Breaker panels do a swap of L1 & L2 phases (the ends of 240vac transformer) for every other breaker position down a vertical row.
In reality the random useage within the house determines how much load is on L1 or L2 phase.
Now for example, If the 120vac loads where exactly equal on L1 & L2 phases to neutral, there would be no neutral current on the neutral line back to the transformer.
Now for a more realistic example, say all the 120vac lights, hair dryer, TV's, etc. turned on resulting in 120 vac loads of 20 amps on L1 side of transformer and 30 amps on L2 side of transformer. There would be 10 amps of current on the neutral line back to the transformer center tap.
240 vac loads like central air conditioner, water heater, clothes dryer, and cooking range create no neutral current as they are directly across the L1 and L2 connections.
Then there is power factor and what it does to current, but we'll leave that for another day.