I can see how this might be a confusing issue.
I don’t know the exact meanings of the terms, but this much I do know…
A monitor with .28mm dot pitch is a sharper monitor than one with
.39mm dot pitch.
A monitor is made like a television set, in the sense that it has
a fine “mesh” (put your eyes up to it), rather like a screen door.
If the wire in the mesh is .39mm (millimeter) wide, then it is wider
than .28mm wide.
The skinnier the wire mesh, the sharper the image.
But, in all honesty, I wouldn’t worry terribly about it. If it looks
good and sharp, then stick with it. If all you have is $125 and all you
find are .39mm dot pitch VGA monitors for that price, while there are
.28mm dot pitch VGA monitors for $175 – try getting an extra $50, but
if you can’t, get the .39mm dot pitch monitors.
The difference is smaller than many nit-pickers like to say.