Just how large are the chances for 300 armour to bounce a gazillion what, 150-200 pen shots? Marginal, its freak RNG par excellence and quite meaningless.
Let's have a look!
The Tiger Ace deflected seven (by my count) shots from guns with penetration 200/190/180. Since both guns were quite close, I'll use 200 penetration. This gives a chance to penetrate of 0.66.
By binomial probability:
Probability of bouncing 7/7 shots: (7!/7!(7-7)!)*(0.66)^0*(0.33)^7 = 0.33^7 = 0.000426, which is roughly one in 2500. Clearly, an extremely rare occurrence.
While we're here, what are the average number of shots to kill late-game generalists?
For the Tiger Ace:
1280 health gives eight penetrating shots to kill and the penetration chance of a point-blank AT gun is 0.66. Dividing the number of shots to kill by the chance of penetration gives ~12.12, which corresponds to an expected effective health against AT guns of 1939.39 (for comparison, the Schwerer Panzer Headquarters has only 1920 health). The standard Tiger is expected to require ~9.85 shots to kill with an expected effective health against AT guns of ~1575.76.
We can do the same comparison for the 800 health, 290 armour Comet and Pershing:
200/290 gives a chance to penetrate of ~0.6897. This gives an effective health of 1160 against AT guns and are expected to require 7.25 shots to kill.
Again, for the 1040 health, 375 armour IS-2:
200/375 gives a chance to penetrate of ~0.5333. This gives an effective health of 1950 against AT guns and is expected to require 12.1875 shots to kill.
Finally, for the 1280 health, 375 armour King Tiger:
200/375 gives a chance to penetrate of ~0.5333. This gives an effective health of 2400 against AT guns and is expected to require 15 shots to kill.