Why I got only 2309 RP even though it must been around 5700 RP???
You were using Rank IV vehicles to research a Rank VI vehicle. That’s two ranks difference and for that you’ve got 60% penalty. That means you’ve got 40% of the base RP.
5773 x 0.4 = 2309
See the RP penalty table here:
More like overcalculation.