I believe I am the right person to answer your question as I have gone through the same problem when I applied through PMSSS.
I got CSE branch in BMS Bangalore in the dummy allotment, I was very happy but in the 1st round, I was allotted Poornima college in Jaipur. This happens because other students who have higher merit than us change their preference after dummy allotment.
If a student having good merit then you will change his college to the college which you got in the dummy allotment then the college will be allotted to him not you in the 1st round. That's why choice filling plays a very important role in PMSSS scholarship.
You read a similar answer on What are the main points to be taken care of while the choice filling
You can see top colleges under PMSSS according to zone, state rank, and stream from here.