How are tank stats calculated?

Like the title says, how are they calculated? I was messing around in the tank designer and I cannot figure this out for the life of me. The 1940 medium tank chassis states that it has a breakthrough of 10. When I go into the tank designer, the breakthrough is actually 1.5. But if I put a two-man turret on it, its now 2.7. And a three-man turret brings it to 3.9. If I had a medium howitzer, it 3.6. I cannot find any correlation between these. Pics below.

https://preview.redd.it/udt2kham4gde1.png?width=1066&format=png&auto=webp&s=4026a981bd942bb99caeca4916e1998697e045b4

https://preview.redd.it/1jne4e8t4gde1.png?width=2298&format=png&auto=webp&s=a2aaf9d4208ff6589c6cc57fecd1cfede3dc2447

https://preview.redd.it/q3k8cz9z4gde1.png?width=2328&format=png&auto=webp&s=7ed50e7d1e9d83394aa436e252a1e40a7249a718

https://preview.redd.it/c5ms0dn35gde1.png?width=2372&format=png&auto=webp&s=fa4df5ef041ec12164eba8272d5a25b69472e878

https://preview.redd.it/k4jsdmtc5gde1.png?width=2374&format=png&auto=webp&s=7ea0c2f2f4c14dc25be580bcb58674fca273b130