Digital BW, The Print

I changed to digital when the developer I had used for my Zone system 
tests was withdrawn from sale. The camera I am making tests with is a 
Casio QV 3000. I'm currently using an Epson 870 with the 
manufacturer's inks and Lyson matte paper.

I tried converting colour to B/W in Paintshop Pro, but I find I get 
better  results by exposing in B/W.
Can anyone tell me how this works? The file size is the same. And
presumably so is the chip. 

Best wishes from Serena .