Are you measuring at the DeviceNet connector, or at the removable terminal block ?
I think the System Power LED indicates the presence of power at the removable terminal block plus an active DC/DC converter to supply the Adapter's logic and the 5V PointBus supply.
The Field Power LED is upstream of the DC/DC converter and shows the presence of 24V bulk power at the removable terminal block, which is then connected to the local POINT power bus.
DeviceNet power is totally separate and isolated from the Field Power and System Power, and should come from a different power supply.