I didn't believe the overall memory usage progress bar, and went through the tedious task of adding up the memory usage of all my applications. My suspicions were correct! The cumulative amount does not match the amount displayed on the progress bar. In fact, the progress bar is drastically overestimating the memory usage. Is this a bug?
Yes, you've found a documented bug! I intend to leave this bug intact, however.
No, no, I'm kidding. This isn't a bug. Allow me to explain.
If you read up on the other sections of the documentation, you'd know that Mac OS X has a very efficient memory managament system. Well there's one thing that I neglected to mention: data often resides in memory even if it is not being used by any application. This is because this memory may be used in the near future, and so Mac OS X conveniently stores this data in memory, so that it doesn't have to take the time to find it again when it needs this memory. One kind of data that is often stored in memory is resources that are needed to launch applications. If you've ever noticed that applications launch faster after already having launched them once, then you're noticing this feature of Mac OS X.
The problem is, there's no specific process that is using this memory. Therefore, where is it supposed to be reported? Mac OS X is lazy, and doesn't report it anywhere, except when it reports overall memory usage. Much of this stored data is in the form of "inactive" memory, and appears as "used" memory, even though technically it isn't being used. So the discrepancy you are seeing corresponds to this inactive memory.