I'm consistantly getting 1/3 the speed I pay for, why?

I initially signed up for 300Mbps when I ran speed tests from multiple sources it showed I was getting 100Mbps. So I upgraded to 600Mbps when that happened it went up to 130Mbps, a pretty small jump. I spoke to a representative and they told me it had to do with my modem. So the next day I bought the top-of-the-line modem and router (separate units). I retested and it was around 150Mbps. So I called again and the representative said she would put a note in the system but as I was on the phone with her I noticed Xfinity was running another offer of 900Mbps for the same price. The representative upgraded my connection without the early cancellation fee. So now I have 900Mbps and am clocking in at 200Mbps. I'm starting to feel like the advertised speeds are just a lie. How can I be getting around 20% of the speed I am paying for? To add to that my friend has Xfinity and pays for 200Mbps a month and he gets 220Mbps, [Edited: "Language"] how unbelievable. So what gives?