These links might help if you read through them:
http://www.yellow-bricks.com/2010/12/20/vcenter-and-memory-metrics/
http://communities.vmware.com/docs/DOC-5600
The DOC-5600 states (which was also noted in this thread by someone else) "mem.usage.average = The percentage of memory used as a percent of all available machine memory."
Basically, Usage is just a percentage representation of active / granted.
That is, if you take the active stat and divide it by the granted stat, the % should be very close the "Usage" value you're focused on. It's basically another way of showing you how much/little memory is really being used by the VM.
As noted on both links, Active memory is an estimated value by the VMkernel based on memory pages touched by the VM. The fact that it's an estimated value enforces the the hypervisor does not have any insight to the Guest OS's memory manager.