There is a clue in the name ADSL, as the A is for "Asynchronous" or, more recently, "Asymmetric" Digital Subscriber Line. That means that the upload and download speeds are different. For consumer lines and because people want to download rather than upload, ADSL maximises the headline speeds on the available wires.
For companies which host busy sites, or which transmit a lot of data, lines are synchronous (usually called DSL or SDSL, as well as many other names for the varying specs of pipes available, such as leased lines, T1 etc) and both ways work at the same speed. Cynical types might also think that ISPs make the download speeds over ADSL slower so corporates will buy more expensive access pipes rather than making do with consumer technology. However, fat dedicated pipes are now so cheap that you can get a pair (for resilience) of dedicated symmetric lines for not much more than a standard consumer link.