ADA4940, the result of OUTP-OUTN

When simulation the ADA4940, found that the Vdiff = OUTP-OUTN , Vdiff swing at 2.5V.?

Why the Vdiff Swing at 2.5V, not 0V? when OUTP>OUTN ,the result is postive, when OUTP<OUTN, the result is negative?