Guest memory alert and 100% reported active guest mem

I have a bunch of Win2012 VM's that receive "Virtual machine memory usage" Critical Alert. It seems that the alert appears when a new VM is added on the host they are located on

The machines have 16GB memory, with "reserve all memory" ticked, and are grouped in three on each host. The hosts are DL380g9 with 64GB installed memory.

I suppose the problem might be related to ESXi reporting 100% active guest memory, but why is this? If this was by design when "reserve all memory" is ticked, why generate an alarm?

I know that the way ESXi is measuring active guest memory isn't very accurate, but reporting 100% and generating an alarm sounds a little too much?

Thanks!