Why is there an 85% rule for taxable SSA disability payment but your software does not compute it in that manner? Why is disability payments taxed at the full rate?

Look at your Form 1040 on lines 5a and 5b to see how much of your SS Disability was taxable.

The amount of SS you received will be on line 5a, and the amount being taxed is on 5b.

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing