Leap Concrete Bridge Design (de calculation question)

Hi, 

I am trying to design prestressed concrete 78" Girder with 8.25" Slab thick. my Prestressing steel centroid is Ypg=9.88 in and total section height H=86.25"

so my dp=H- ypg=86.25-9.88=76.37". I have no mild reinforcement in the girder, so dp=de. The program is calculating de to be 83.03", and i would like to know why the reason for that?

Per AASHTO 9ed 

Thank you 

Ashvin