Think of it like a pipe. With ethernet, the pipe is always the same fixed diameter. With WiFi, your pipe is constantly changing diameter, multiple times a second, sometimes its bigger than a ethernet pipe, sometimes its smaller. Sometimes its much smaller, or even pinched off completely. Now think of the flow of water coming out of the wifi pipe vs the ethernet. It will be stopping and starting, spurts and surges of flow, then nothing. The flow out of the ethernet pipe will be a constant steady stream.
Ethernet is a reliable and dedicated link. Wifi is very much a shared medium subject to all sorts of influencing factors that change its performance and availability. If your wifi works, then great, you got lucky, that’s all. Ethernet will work as long as it’s plugged in.
Regarding the 100 vs 1000 issue, ropieee sets the lan port to 100 vs 1000 on the rPi3B+ because it causes issues to the USB controller when running at GigE.
100Mbps is at least an order of magnitude greater than the needed capacity to stream hi resolution audio, two orders of magnitude just aren’t needed.