As with any internet service provider, CenturyLink’s internet speeds can vary quite a bit based on location and other factors (wired/wireless connection, modem type, router type, router location, materials used to build your home).
Your neighbor could qualify for up to 940 Mbps fiber internet, while you might get only up to 20 Mbps via DSL. The “Up to” means that CenturyLink doesn’t guarantee you’ll get 20 Mbps (or 940 Mbps, for that matter), due to the factors mentioned above.
So, if you’re getting only 5–12 Mbps on a 20 Mbps plan, CenturyLink wouldn’t consider that a problem.