Hi there,
In a virtual environment, metrics received from inside the guest can be unreliable for a number of reasons including hypervisor overhead not being taken into account, clock skew and more. This blog post has some detailed explanation http://vpivot.com/2010/02/10/inaccuracy-of-in-guest-performance-counters/
This is why it is extremely important to monitor the hypervisor metrics like Virtualization Manager does.
You also have effects like "Transparent Page Sharing" (TPS) where VMware can reclaim memory across a bunch of VMs running on a host.
Jon
Memory management in VMware is a complex topic - you can find more details here http://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdf
Jon
