I am not sure sure which tank(s) you mean by “weird ass stuff that barely or never existed” but, in that paragrapgh, it looks like you apply it to the IS and IS-3. Which is weird because both of these tanks were produced in great numbers, greater than Tigers. With one caviat - for the tiering purposes, the game rolls two “real life” tanks (IS-1 and IS-2) into one and calls it IS. Same as it does with KV-1 and KV-2.
And IS-2’s most definitely were “actually fielded by an army in WWII in battle.”
So yes, while the IS-1 was mostly a prototype (well it was a production model but it had such a bad gun, they cancelled it very early), the tank we are driving in the game (with the 122 mm gun) was called IS-2 and they produced over 3,000 of them, most of them during the WW2.
IS-3 was also a production model, they produced over 2,000 of them, however, officially, they did not fight the Germans. Some of them may have fought Japan in the summer 45 (I guess there is no proof for it or something).
As for the Tigers, about 1,300 of Tigers were produced and about 500 of Tiger II’s.
As for the tanks’ in-game performance, it’s not really fair to put Tiger and IS-2 in the same tier and keep (more or less) their historical stats. The same applies to KT and IS-3. IS-2 was created as Soviet’s response to the Tigers, naturally it was more powerful.
But that’s when realism and games don’t go well together. Countries did not develop their “tiers” of tanks at the same time in perfect harmony making sure similar tier tanks would have similar “fair” stats. To put it simply, Tiger was created to beat Soviet KV’s, IS-2 - to beat Tigers, KT - to beat IS-2’s and IS-3 - to beat KT’s. Every iteration is supposed to beat what the enemy has, otherwise what’s the point.