That's because of the way these files are 
normally set up. Every few seconds there's a key 
frame (not related to a key cel ;-).  These key 
frames are normally the ones that can be 
screencapped, while the ones in between can't.  
Normally, I find that when I pause a player on an 
frame in-between two key frames, then click on 
some other window, then come back to the player, 
the movie file has skipped ahead a few seconds to 
pause on the nearest key frame rather than the 
one I wanted to screencap.  If you look 
carefully, you'll probably see that the images 
you ended up capping are *slightly* different 
from what you thought you'd paused on.