ISPs will generally not sell you a plan that advertises throughput in excess of the achievable modem sync speed, unless your achievable speed is already below their lowest tier of service. AT&T won't let you sign up for 18Mbps DSL if you live 2km from a DSLAM that doesn't even have ADSL2+ equipment. The "up to" provision is because their backhaul capacity is less than the sum of the last-mile connection speeds.

My last mile is 1 GBps fiber. I have a choice of a hundred or so ISPs over that fiber. The cheapest ones drop to single-digit megabit speeds during peak hours because the ISPs have such oversubscribed interconnections.