When assigning group policies using this method the Meraki kit watches http (and only http) traffic to identify what the client says it is using.
Typically group policies applied in this way can be slow to apply as a lot of web traffic is https based and being encrypted it can't see what the client it says it is. It has to wait till it sees that http request.
This method doesn't tend to be very reliable. So don't use it for strict enforcement. Use it where it would be nice if it detected the OS correctly but not a big issue if it doesn't.