That is one of the points. Some powered USB hubs supply power only with the intent to supply full power to each port. Other powered USB hubs will continue to power each port at full power even when the computer is off.
For instance: The USB cable that goes to the computer only can draw "x" amps; Lets say 0.5 Amps (whatever the value may be). In this scenario the 4 Ports on the hub needs to share the 0.5 Amps, whereas the powered hub can supply 0.5 amps to each port (full power).
It seems the hub you have defeats the second purpose of the powered USB Hub (being able to supply full power and power regardless of the host connection state).
You also may want to check the AC adapter with a multimeter to make sure it has proper output (as stated on the label).
Edited by OverK1LL - 1/4/11 at 7:28pm