Some cameras are "programmed" to be cautious with exposure & in certain situations will underexpose to protect highlights - my Samsung GX-10 does this, not sure if the D80 has similar programming but...
I use exposure compensation - take the first shot, check preview with histogram - dial in positive compensation depending how far to the left the Histogram is - take the next shots & recheck
One other very useful technique in these situations is to use "bracketing" - if the shots are quite dark +/- 1 stop would be useful
simon