For a long time I have observed the speed of my Vodafone HFC Max (900Mbs down / 100Mbs up) vary wildy.

At times it tests at 700Mbs, others 350-400Mbs. Sometimes in the space of minutes.

I use the Speedtest app from Ookla in Windows 10 to do the testing. I am on a wired 1Gb connection. 

Connecting to the Vodafone Wellington  test server give consistently slower results than Citylink or XTreme networks.

If I connect to a VPN, the speeds rise to 700-750Mb/s pretty consistently to the test servers. On occasion the Vodafone test connection drops to about 350Mbs on VPN, but usually subsequest test show it rising up to the 700Mbs level.

I have run these tests using the Vodafone supplied Ultrahub and my Edgerouter SPF X.

Neither seems to give a clear better result. They both bounce around in speed a lot.

My main query is why the VPN shows consistently better speeds than when not using it?