Here is the missing context :

( this is a validated Meraki design btw )
We have sites that have 2 MXs.
MX A routes red vlans
MX B routes green vlans.
Switch "sees" / samples everything.
When you perform GetNetworkClients on Network A , since it contains the switch you will get EVERY clients even if they are not routed by MX A. This is something you CAN exclude from the client page on Network A ( from the dashboard ). But you CAN'T do that with the API.
Hope that helps.