if a bond is trading a premium nominal yield
If a bond has a 7% nominal yield or coupon and was purchased at a premium of $103 ($1030), then your YTM will calculate lower because the 7% interest is only paid to the $1000 par. The $30 premium does not earn interest and is not redeemable at par. So, a 7% bond at a premium is not really yielding 7% in that example.