I suggest you check the data in vCenter. Click on the VM, switch to Performance tab in Advanced view, then switch the shown graph to Memory, go to the advanced settings and enable only the Usage counter. If the graph shows values similar to those in Virtualization Manager, everything is fine. If not, upgrading to the latest version of Virtualization Manager might help - I looked at the same counter in our lab and everything seems to match for the VMware and vSphere versions you mentioned. Please tell me what you find out.
Side note: I think one possible reason for a mismatch between the graphs in Virtualization Manager and Windows could be caused by memory ballooning. The balloon driver could be reclaiming some memory from the guest system, making it look occupied to the guest system. I'm not sure how this is handled by vCenter, but it seems logical that it would report only the memory that is actually used by the guest system without including the memory balloon. Including it would make the VM's memory graph depend on the overall memory consumption of the host, which, in my opinion, wouldn't make much sense.