Mismatched VM memory at host vs. Xenserver level

We noticed a number of virtual machines were reporting a different amount of memory at the VM level than what is shown at the Xenserver level. In Xen the VM shows something like 1024 or 2048 but the "free -m" command on the VM itself shows a value like 991.449 or 1993.56.

I have tried to change the memory via Xencenter and Xen command line but it doesn't affect the VM reported on the VM OS side.