Why are broadband speed always shown as maximum speed?

This is because broadband speed is affected by the variables outlined above, ISPs are unable to guarantee a certain speed. So they are allowed to quote the broadband speed as the maximum possible speed. Hence the term "best effort".

This Q&A was found on iBurst 4 U : http://www.yburst.com/izzi/modules/smartfaq/faq.php?faqid=2