AFAIK, contention ratio is itself designed on the principle that all the users would not use the same pipe at maximum bandwidth at the same time.For eg an ISP allocating a 5 Mbps virtual circuit from a particular DSLAM to the ISP backbone and sharing it with 50 users. Now if 25 users turn out to be full bandwidth utilisers it would mean that there is a good chance that at any given time they would run the connex at the max speed available to them, thereby affecting the bandwidth available to the other users of the same 5 Mbps pipe.However, this explanation is not to endorse the FUP, as ideally, the ISP should increase the 5 mbps pipe and not cut down speeds of users coz each customer has the right to use the connex to the max coz that is what he is paying for. So FUP is an easy strategy employed by ISP's to bail themselves out of bandwidth problems without investing in or upgrading their network.