So this is my internet speed :

Not that bad right? This was also taken whilst some-one was using netflix in my house.
So why is it that when I join something like CS:GO that my ping fluctuates from 30 to 150 constantly causing me to rubberband? It is really annoying. I've been assuming that it was just my family using the bandwith with netflix and such, but the speedtest was taken when they were using the bandwith and the speed is fairly good.
Anyone have ideas on what's going on?