hello thushar.

you can think of the situation as the reservation for obc category are very less to the general category seat infact if a student get the same rank for the obc category and another get for the general category the best chance is for the general category as the seat reserved for the obc category may be occupied by other applicants who had better ranks than them. moreover if the student who has 15000 general ranks belongs to obc then his obc rank will be about 500 which increases his chance of admission in good colleges..

hope this information was useful to you..

all the best for your bright future..