When the Nav Light switch is on, both the Red/Green nav lights AND the anchor light come on. When the Anchor light switch is thrown, just the anchor light is on.
Take the entire panel off and trace back the wires, it should make more sense
One pair (Ground and Positive) to anchor light.
One pair (Ground and Positive) should connect to the output side of the anchor light.
The fifth looks like another ground. It could be to light the anchor light switch. When the Nav switch is thrown, the anchor light switch should light, even if the switch itself isn't thrown.