Vero 4K with 1GbE adapter iperf3 results

Playing 4K content from my NAS using fstab over wifi I get buffering every so often, much less when wired over 100MbE, maybe once or twice for a second or two on some films, most recently Ready Player One. Using an Anker 1GbE adapter there is no buffering but have the following query regards iperf3 results…

With iperf3 running as a server on my NAS and the Vero 4K as a client I get a near constant 185Mbps, then running it the other way around with the Vero 4K as the server I get a near constant 320Mbps.

How come its different speeds going different directions?

Very often it depends on the ethernet hardware and physical buffer setup in the hardware.
My experience is that with intel-chipsets (against broadcom etc.) you get symmetric results.
With broadcom usually you can download faster than upload.