It's the same cause as the
other circumstances we talked about. Due to the angle of fire you're still likely to miss because it's barely grazing the corner. The color predicting whether or not you have line of fire can't be sure of that (since it's still possible to hit, just less likely than your actual accuracy). Sentries are large but still don't fill their entire cell like Behemoths do (but they still take up more space than Grunts, and much more than Swarmers).
