You purchased the car, completing all requirements Sale deed, Letter of sale was made on your name,
You didnt get it transfered on your name, Suddenly you want to sell the car, Now the sale deed is made on the name of third owner,
In exercise office, The car would be first sold to you, transfered on your name and then on the name of the person you sold your car too, (i.e 2 transferes, Shortly reffered as double transfer).
If you had purchased the car on open letter, (letter of sale not mentioning your name) It could be directly transfered on the third owners name as the second owner.
I hope you understand.