Digital BW, The Print

Hi,  I cant help but think I should know the answer but here 
goes.... When I use QTR and print at 720 I get pretty good results 
in relation to my original.  When I move to 1440 I am roughly 2 
stops darker.  I realize this is laying down more dots but 
shouldtn't it be close to the same brightness at either setting?  
What am I missing... Thanks BG (using epson UC's with a 9600)