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.
Then why not 320/115 for single one and discount for double call-ins?
After all this is P4 level tank that will come much later now.
What is the reasoning behind putting P4 level tank 1 CP before IS-2/Tiger?
To me that doesn't make much sense.