Well, you must consider game is still "work in progress", so you can guess graphic artists are tuning colour for best result (both for artistic effects and about easy readability and right contrast).
Second point is screenshots come from different sources, some directly from screen capture, others from a longer path of screen capture, print on game magazine, scan acquisition.
Some are probably been retuched in contrast/lightness for enhance the details on small format...
My humble opinion is don't bother at this time of minor difference, until final screenshots start to appear on official site