Why this source transformation leads to different result?
You can't convert a voltage source to a current source the way you have done it because the 15 kohm in the middle limb becomes connected to the output (node a). You have to work from the left to the right. In other words you broke the rules of conversion by ignoring the 15 kohm's circuit position.

However, just stop and think about the circuit - the output connects to a 15 volt source via a 5 kohm resistor - all the components left of the 15 volt source are therefore meaningless to the analysis.