|Check out wikipedia|
In WoT you are not playing against an all knowing general or computer, but against a disparate group of Europeans spread across 20 odd countries and age ranges. As such, ability, coordination, tactical awareness and stupidity will vary hugely.
The medium, although vulnerable, is perfectly place to exploit these gaps with it better speed it can make the breakthrough required.
It doesn't always work though, but a tier 7 tiger and a tier 8 medium have to work VERY hard to beat a tier 9 E-75 prototype next gen German post war tank.
VERY HARD! Enjoy!