It's a user customizable setting. There are various reasons why modems seem to have good speeds and DSL don't

1) some ppl. select purposely Modem even though they have a much much faster connection (including myself).

2) ppl. tend to ignore setup wizards (modem gets selected automatically)

3) ADSL isn't per se fast . The A in ADSL stands for Asynchronous and means different speeds for up- and downstream. Usually fast downloads but slow uploads. For example 1MBit/256kbit, 4Mbit/2Mbit. ADSL has also the drawback that download speeds drop to about the same speed as the upstream if the upstream is maxed out (a fix is to adjust the upload bandwidth slider and/or to select a lower speed, see 1).

4) Real modem users tend to select a higher speed in the hope to get better download speeds!?

5) On the other hand are DSL user often connected through Router. Configuring a Router properly is a science!

6) ...