If your car managed only a quarter of the advertised top speed, you'd go ballistic! Yet in the world of UK broadband internet, customers face an array of ISPs making claims of download speeds of up to 8 megs which (via ADSL connection) turn out to be about four times what is technically feasible given the existing infrastructure.
Do they complain about this? Of course not, because most customers are non-geeks and believe what they're told if they don't know better. Their main sources of complaint involve customer service issues or connection breakdowns. But eventually it will not just be bitter and twisted individuals like me (who looked into the truth about ADSL some five or six years) who are aware of the reality. The truth will out and all trust will be gone.
Managing expectations does not mean blinding with science. It's good enough (plus) to promise excellent, but not impossible, performance and to deliver it.