Churchill and kv 1/8 is a design choice to have more rear armor, for kv2 it should be same as is2 and for isu152 same as elephant
"Heavy armored Tanks
Developer Comments: We felt the risk associated with flanking a Heavy Armored Vehicle was much too great because of the high rear armor. To help balance this risk vs reward we are reducing rear armor on all heavy armor across the board.
Soviet IS-2 rear armor reduced from 205 to 140
Wehrmacht Tiger & Tiger Ace rear armored reduced from 180 to 140
Wehrmacht Elefant rear armor reduced from 150 to 110
OKW Jagdtiger rear armor reduced from 150 to 110
OKW King Tiger rear armor reduced from 225 to 150
British Comet Tank rear armor reduced from 180 to 130"
It is not a designer choice it is an oversight, because those unit where hardly used.
PzIV with 110 penetration has 61% chance to penetrate rear shots vs 180 armor going to 69% at point blank. The chance is simply too low.