OK, so I made a quick test. Test conditions:
Kingtiger placed on the map, being "neutral" (neutral units have same stats as units belonging to one or the other faction but they don't move nor attack) then I built 17pounder gun and via "Attack move" order I let it fire frontally at max range at the KT - no AP active nor shooting from camo.
First 8 shots in a row deflected, 9th shot penetrated (took about 1/3 KTs HP), next 7 shots in a row deflected, 17th shot penetrated (again about 1/3 of KTs HP), next 6 shots deflected, 24th shot penetrated - KT destroyed.
This was a single test but 3 penetrations out of 24 shots doesn't seem to confirm your report that 17 pounder always
penetrates frontal armor of KT. Also the 17pounder has a penetration bonus of x1.25 (+25%) when it fires from camo so at max range against KT that makes 18.4% which is still not much. With AP ammo (when activated when in camo) you get to 28.9% at max range which is significantly better but still far from 100% penetration you mention.
It seems that this is another example of the phenomenon that is quite frequent here - people don't remember the 10 times when their tank deflects incoming shot (because it is expected and thus a trivial event) but always seem to remember the 11th shot which the penetrates tank (because this is unexpected/surprising).
Have you ever heard the "theory" that when you put a butter on a bread and drop it, it will ALWAYS hit the ground with the buttered side? There have been tests that report that the bread actually hits the ground with the buttered side and unbuttered side in roughly 50:50 ratio but when the unbuttered side hits the ground, you can pick up the bread, put away the dirt quickly (because it doesn't stick to the "dry bread") and eat it so it means no extra work for you. But if the buttered side hits the ground you need to remove the butter (because dirt sticks to it and you cannot clean it from it) and butter the bread again - that means spending extra time and doing extra work. Because of the extra activity the brain remembers the event better and thus creates an illusion that the buttered side hits the ground every time. Here it is basically the same thing - when KT deflects shots, it is expected because it has strong armor and deflected shots create no more work for you, so you don't remember every deflected shot. But if KT gets penetrated or destroyed you need to build a new one which costs you resources and time so you remember better every time when something penetrates the unit and thus it creates an illusion that it always happens...we really need to give a name to this thing