As I've blogged about before, System Activity now has an option to show the detailed memory usage of a process. However the numbers shown there don't match up at all with the numbers shown in the process list. This can be very confusing.
The reason is because the process list shows an approximation, so tends to underestimate the Private memory usage, and overestimate the Shared memory usage. It also doesn't help that it had a different name in the tooltip (URSS etc).
So I've added WhatsThis and tooltips to everything to try to explain the situation, and tried to use consistent naming. Hopefully this help. I've also updated the userbase FAQ.
Please let me know if you have any questions that are not answered on this FAQ. Feel free to use the discussion page there.