Initially at their base vehicles have the same stats for each nation, yes. But depending on your (for the majors) contry specific Design Companies you alter their initial stats. Ontop of that you can also create variants of your vehicles by increasing their aspects with experience.
On the field it is unlikely that an german pzIV has the same stats as an russian T-34 or american M4 Sherman, even tho they started out with the same base stats at their core.
Even as germany you might not have the same pzIV every campaign. Depending on what company you pick to desing the tanks like Porsche, Henschel or Man, the stats will alter and the tank will be focused around different aspects. They alter their stats upon research permanently and can be switched out for different vehicles. You might have MAN pzIII, Henschel PzIV, Porsche PzV and Henschel PzVI Tiger I for example. Any active company can build any design tho. Porsche can build Henschel Tiger I tanks just as Henschel can build MAN PzIV for example.
Here i got an example for you that i made a while ago:
Note how the Scharnhorst has less firepower due to its smaller caliber of guns but also has an higher topspeed and is less visible. The research time is different due to modifiers to the base research time.