Measured speed is not accurate

I am based in Ireland as Virgin Media customer, my paid service is 360mbps.
If I do a speedtest with NSpeedTest, the selected server will be Vodafone/Dublin.
The measured speed will be between 70-90 mbps (I did a good few test...) 
If I check the speed over browser on Speedtest.net and select the same server The results were between 220-260 mbps. 