[nextw33k]

This is a well documented problem and the crux of the net neutrality debate:

http://mattvukas.com/2014/02/10/comcast-definitely-throttlin...

Netflix buys backbone access from a backbone provider, the ISPs then have a connection with that backbone providers. The pipe from that provider to the ISP gets congested and there is an argument as to who should pay.

However the pipe to your VPN data centre and Netflix's backbone provider are congestion free so you get a faster rate.

Internet routing isn't as dynamic as we imagine and whilst Netflix offer to put caching boxes in the ISP network, the ISP's are not keen on that solution.

End result you are performing a manual re-routing of data to bypass congested links.