Source of ESX PacketsDroppedRx ?

I've recently discovered a few ESX hosts (ESXi 5.5.0) in order to evaluate whether or not to discover and monitor our entire ESX environment.  Since day 1, both have been in yellow error condition for the datapoint "PacketsDroppedRx" under the ESX Host datasource (rate fluctuates between 25k and 125k per second).  However, for datapoint "DroppedRx" under ESX Host Interface, the total indicated rate for ALL vmnic instances is always less than 10k per second, and unfortunately there is not a corresponding "DroppedRx" datapoint under each instance of ESX Virtual Machines (there is a PacketsTx and PacketsRx for each Virtual Machine but not one for drops).

1. Strangely, our ESX Admin can find no indication of this "PacketsDroppedRx" value in vSphere anywhere and he is not aware of an ongoing degradation of service that might be expected due to excessive dropped packets.  So I have 2 questions:

• 1)does anyone know to what "PacketsDroppedRx" can be attributed, or at least where to look under the ESX hood to find it?
• 2)does anyone know why there is not a direct relationship between "DroppedRx" on vmnics and "PacketsDroppedRx" on a host? -- I would expect that the sum of the each individual vmnic "DroppedRx" would equal the value of "PacketsDroppedRx" for that host.