Sorry for the late reply but I'd like to give the information anyway... The WPS and the imageviewers (as well as the pictureflow plugin) use two different methods to draw the picture.
In the WPS, the picture is drawn by using the 4 grey levels that our targets' greyscale displays provide (black, dark grey, light grey, white), there are no intermediate levels and areas that have an "in between" brightness will be dithered; see
wikipedia article on dithering.
The plugins use the so-called greyscale library which cheats the eye by turning a pixel dark and white real quickly so it appears as grey to the viewer, the tone depends on how often it is dark or bright in the sequence. It can achieve up to 129 different levels of grey.
As you can imagine though, the greyscale library has quite some computing overhead and updates the display very often (even on still pictures) - both take quite a lot of battery whereas the WPS method just needs a one time computing and drawing of the picture with no further display updates. That's why it's unlikely that the WPS will get the greyscale library way at any time.
P.S.: I just thought I'd throw the idea out without trying myself though: It's possible that you could get more pleasing results if you don't rely on the Rockbox's internal dither mechanism but prepare all your pictures to use the 4 greys with a PC's graphic program (white, 1/3 black, 2/3 black, black).