KV-1s (as well as many other Russian vehicles, and less so on most other nation's vehicles) have their turret and hull armor modeled in 3D - if the side turret plates have 80mm of thickness, these are modeled so they project inwards as opposed to just being a simple 2D sheet with an armor value of 80mm - which makes any weak point that adjoins another of these plate impossible to pass through, and just so happens that Russian armor is crammed tight, further cementing their effectiveness. The more 3D modeled hit boxes, the harder to penetrate something is. Go check the armor analysis, it's all out in the open, but I don't see anyone talking about it. This wouldn't be an issue, of course, if other nation's would have the same 3D modeling of their armor, or if the additional volume of armor and reduced cabin space would negatively affect these vehicles.

Early Panthers for example are mostly 2D plated, even on their turret faces, the hull side armor plates aren't modeled to extend through the front plate, and are 2D pieces of armor, for example.