That illustration of yours that states projectile: 100mm/L52 APBC, target King Tiger hull and then shows the green part. That side is bugged as it seems.
Lt. Wolfgang Kloth of the 2nd Panzer Div. commented that German crews quickly realized that the IS-2 had to first level the gun downward to reload the cannon, then reacquire the target each time to fire.
This is a fact and Germans used that to their advantage.
It wasn´t about the total kill claim, but the ability of the Tiger II to defeat IS-2s.
"During the first engagements in early 1944 the IS-2s showed an unexpectedly high amount of losses, especially against the smaller german 7,5cm guns of StuG and Panzer IV aswell as the similar 7,5cm Pak. In many tests and trials of fire made by the design offices, the cause was found in the unfavorable stepped shape of the frontal armor. this was 60mm thick at an inclination of 18° flat and then stepped up at the drivers hatch to 120mm."
The shot would get deflected down at the drivers hatch into the 60mm strong armor.
(Only found it on the German wiki, but you can use google translate if you dont believe me. it´s in the first section of the article.) http://de.wikipedia.org/wiki/IS-2_%28Panzer%29
Read the text below on the image. Green means "no penetration" while red means penetration is likely.
Interesting where those germans got their information. Soviet manuals say that the gun can be reloaded just fine in all positions.
The D-25 gun has a separate charge, so it is important to ensure complete ramming of the shell. In order to load the gun, do the following:
Disable the inertial guard in case a shot has not been fired.
Press the breech lever to disengage the breech lock from the loop on the frame.
Move the lever back and to the right as far as it will go.
Place the shell on the tray, disengage the tray lock, and move the tray to the cradle.
Push the shell from the tray onto the guide and use the punch to ram the shell into the breech such that the driving band engages with the barrel rifling.
Insert a propellant charge into the chamber.
Close the breech and place the safety into the "fire" position."
Nothing about angles is mentioned.
I think it's better to trust soviet instructions rather than some german "observations"