Both SEBC and EWS reservations are different and they cater to two different groups or Categories. SEBC is a reservation for Economically Backward of OBC Category. But EWS is a reservation for Economically Weaker Section of General Category. For claiming EWS reservation, you should not be claiming any other reservations pertaining to OBC/SC/ST. So these are 2 different Category reservations altogether. But if you look at UPSC cut offs, EWS had lesser cut offs than those of OBC. Cut offs depend on various factors. If the availability of seats is lesser, then the cut off increases. So we cannot point out one reason and name it as whose cut offs might be lesser or whose cut offs might be higher. It depends on the Highest score in the exam as well.