Paying your bills on time accounts for about 35% of your credit score but there is another 65% which includes amount owed (30%), lengthofcredithistory (15%), new credit (10%) and type ofcredit (10%).
Opening a new credit account then reduces the lengthof your credithistory, which is about 10% of your credit score, by lowering the average age of all your accounts.
Your credit score is based on your payment history, outstanding debt, lengthof time you've had credit, types ofcredit you now have and the number of inquiries.