I could not follow the step for prior year installment sale (6252).

My installment sale 6252 information was transferred from prior year sale in 2018. Line 18 contract price (calculated by PS) on form 6252 is equal to gross profit on line 16 (calculated by PS); therefore, my gross profit percentage is 1.0000.  Why is contract price equal to gross profit ? Instead it should be the same as line 5 selling price.

Thanks in advance for your quick response.