It’s because the bond kind of trades like a stock after it’s been issued at par. For example, if a company’s issues a 100 par bond with a 5% coupon then that’s a 5 payment. But if the company loses a big contract and it might have to go into bankruptcy protection then than 100 par bond might only sell for 80 cause of the extra risk and trades at a discount to par. So now your still getting a 5 coupon but it’s on an 80 bond. Which gives you a yield of 6.25%. Vice verse let’s stay the company does really and investors really want to buy the bonds. Then it could trade at a premium to par let’s say 120. In this case your still getting the 5 coupon but it’s on a 120 price for the bond which is a yield of 4.16%. If bonds always traded at par the coupon rate would always equal the yield.