What you say is mathematically not possible.
Or you should start playing some lottery asap.
Statistically unlikely, but not impossible. Hell, it's theoretically possible for American Engineers to beat Vet 3 MP 44 Stormtroopers. But only theoretically, the dice-rolls required for that to happen are so unlikely it'd be a universal anomaly, like a billion to one (maybe exaggerated, but you get the idea).
Statistically, the IS-2 should have won. Looking at the numbers, it would have taken 3 hits for the IS-2 to kill the weakened Panther. Assuming it is attacking the Panther's heavy frontal armour, there's a 60% chance to penetrate. The Panther on the other hand has to land 6 penetrating hits on a full health IS-2, with a 50% chance to penetrate.
In other words, that Panther should have been dead twice over, the only reason it survived was by RNG.