I have the same thing for my VPN and I don't know the reason but it is NOT faster. It is just a quick second boost nothing more. I have a 3mb/s upload and on my VPN on speed test it says I have 10mb/s. I don't actual have it and can not reach that but for some reason speedtest seems to say so.
I'd say it is a mistake that happens with a program such as speedtest and how it conducts the test.
Your VPN does not bypass throttled speeds from your ISP.