Hey Rajkumar,

Neither you nor the experts here are going wrong.

The minimum eligibility percentage requirement of DU for B.Sc (hons.) Chemistry says that-

The candidates must secure 55 percent or more in aggregate of 3 subjects (i.e. as you said of - Physics, Chemistry and Maths) and must pass in one compulsory language i.e. English with 50 percent marks.

Hence four subjects are ultimately taken into consideration. For further information you can check the official website of DU too!

I hope your doubt is cleared! :)