Your logic is correct. T34/85 and E8 are "upgraded" versions of the normal medium tanks, so, they should have more health. Ok. I respect that.
But then why the P4 ausf "J" doesnt follow the same idea? It has 640 as the normal P4.
Panzer IV J was a downgrade that should actually be a worse tank overall.
It'd actually be kind of cool though if they gave it slightly higher HP (maybe 720 like the M4A3E8), but far slower turret traverse and worse visibility.
That or it could be cheaper without the bonus HP, since that was the intention of the J series anyway.