Sometimes nothing. Sometimes bad gamma. Sometimes bad lighting. Sometimes there is no screenshot. Sometimes the screenshot doesn't capture the beauty of the level.
And Tronyn gets the shots right in every way.
This shot of JPL's map is particularly good for example:
http://www.quaddicted.com/feature/tronyn/gth2.jpg
Sometimes a map author exerts and pushes so hard to get something out, they don't have anything left in the tank for fiddling around to get a screenshot perfect (or even make one).
And on other occasions, maybe the map author is great at making the map but isn't so proficient with a photo editor or client settings.
I'm not faulting map authors whatsoever, I'm praising Tronyn and saying he has a great knack for finding the right spots in a map for the presentation of a level.