The order in which the various network interfaces connect is not really important. Even though WiFi connects and get an IP address before another interface, does not mean that traffic will flow through that WiFi interface for that reason.
The service order essentially means that if you want to establish an outgoing connection and have two routes of identical specificity towards the destination, then the service order determines which route to take.
Some examples:
You open a connection for "google.com" and the most specific route for that is your default gateway. You have one for the WiFi interface and one for the Ethernet interface. As you have prioritised Ethernet higher in your service order list, the data will travel out the Ethernet port.
You open a connection for "192.168.10.1", a NAS server that is connected directly to your WiFi-router. The most specific route is for 192.168.10.0/24 via your WiFi router. In this case, the data will travel out via WiFi.
Another computer connects to your computer via WiFi for a file transfer. Your computer knows that the other computer is accessible both through WiFi and Ethernet - however the service order does not come into play here - the data is sent back via the same route the connection request came through. That is, the file is transferred via WiFi.