Like you can see on the attached screenshots, when you stand in front of a mob (and you have activated the visible hitboxes = F3 + b) you will see that the line seems to go diagonally in the sky, which is not right, because the zombies eye direction is horizontally.

Also when you press F5 twice and then try to see the blue line, it goes to the cross (sometimes only!) and also starts on the left side of the screen (or of your head) instead of starting in the middle.