also sometimes the bottleneck is only in one specific location or component.
You may recall macbooks that could take 3gb but not 4gb of ram. Both slots could accomodate 2gb sticks each, but the bus could only address a total of 3 so installing two 2gb sticks would "see" both of them but could only use half of one.
The limitation can be one of OS also. Look at the windows madness where they were selling dells bundled with upwards of 4gb of ram that couldn't address it all in the software. (the real crime there however was that for all intent and purpose windows said 4gb was installed and appeared usable, but was not)
