The start battery negative is typically connected directly to the engine and not through the shunt first. So there will be a draw when the combiner is combined that is not measured on the meter. Typically, this is not much of an issue since the start battery takes almost no charge current when charging.

It is possible to connect through the shunt, but for us that would entail adding a 50-60' round trip run of negative cable, along with massively upsizing that cable. If your shunt and house batteries are very close to the start batteries, then it would be easier.

Mark