Differnt mbps in modem and speed test?

When I look into my modem in the homepage it says my upload speed is 8125 kbps, which is 8 mbps. when I do a speed test i pull 5.8 mbps. I pay for 6.0 dsl. not complaining about the speed I get and what i pay for but when I use multiple devices speeds are slower. I understand that speed will slow down doing this but I would like to know where the other 2 mbps are? I have a 2Wire modem and a motarola and both give the same 8125 kbps in the the homepage.|||that 8mbps is the maximum speed, which you will not get because actual speed is much lower.|||That's NORMAL, you'll NEVER get exactly what you pay for.. I pay for 3 and get 2.53 !