Guest VM shows 90% memory usage, doesn't match vSphere

We have a monitoring software that is alerting that a virtual server has high memory usage, when the admin of that server logs in to the Windows Server 2008 box he see's windows is reporting 90% memory usage:

However when we look at the Performance tab in vSphere we see that vmware thinks the VM is only using on average 12% memory:

What explains the difference that not only I can understand better but also explain to the server admin and management as to why the huge difference in numbers?

Thanks in advance.

Steven