The reasoning is quite simple, there is a greater opportunity cost associated to a double call-in given you have to save up 240 fuel. For that reason, the double call-in is provided at a discount. The cost of the single call-in T34-85 better represents what its cost would be if it was integrated into the core tech.
Bottom line, saving up 240 fuel has an associated opportunity cost; therefore, we provide the user a slight discount to compensate for this factor.
This makes perfect sense.
That said, I predict the double T34/85 doctrines becoming somewhat obsolete after this patch, which is fine by me given the new commander but it's something to be aware of.
Moving the double call-in to CP9 is essentially saying that the double T34/85s are equivalent to a German heavy tank call-in (and more expensive than the Tiger), which they really are not. (To understand this better, imagine how popular a German doctrine would be if it offered double P4s at CP9 instead of a Tiger at CP10...

)
The reason the double call-in doctrines were popular before this patch is that they offered an excellent medium armour option (something the Soviets lack completely) without having to build any buildings,
provided you had dominant map control and/or were prepared to risk using AT guns and guards and/or AT nades for your mid-game AT. Unless you denied the German player fuel for most of the game, this was fairly risky even at CP7.
I understand that your view at Relic is that call-ins should supplement tier-based tank options, which also makes sense. However, that being the case, double T34/85s make very little sense now. If you can't use them as your first tanks, they'll likely arrive when German heavy armour is already on the field (you need to spend fuel on other things to avoid being overrun in the mid-game), at which point they really underperform for their price. Other options (IS2, single T34/85s, spamming T34/76s or SU85s) are far more attractive at this point.