Its all random. Literally up to the RNG god to decide who comes out on top. If I could be bothered, I'd find a 1v1 IS-2 vs. Tiger duel I had the other day in my stream VODs. It wasn't as controlled though, I was continuously reversing my IS-2, always showing my front, so this affected accuracy on both sides but they were both engaged the whole time (never out of los, no range issue etc.). My IS-2 eventually won. Its literally how lucky you get. Its just as possible the tiger may have got 3 or 4 crits in a row and won.
You can outplay someone, you can have sometimes luck, but Tiger is stronger.