One thing I've picked up about running VMs is, you can't trust the RAM utilization reported by the guest OS (the VM.) It will *always* claim to be seeing the max available RAM that was granted in the hypervisor. Instead, on the Hyper-V server (this is easier if it's a full OS) there are specialized performance counters in PerfMon for tracking the RAM used by a VM.
I believe with the dynamic RAM settings, when "actual" RAM isn't being "actively" used by the guest OS, it's swapped to disk (so the VM still has X amount of RAM, but Y amount has been swapped to disk) rather than actually "removed" from the OS.
As for the "Assigned Memory" field, I *think* that might be how much "real" RAM on the Host is currently being used / given to the VM.