Why torsional constant of composite section is lower than part 2 deck section?

The Ixx value shown in deck part is simply the Ixx value of rectangular section calculated by the SPC.

But when the Ixx of steel + deck is observed, the Ixx of deck is reduced by Ec/Es as the modulus of steel and concrete is different. Thus, the Ixx value shown in deck is reduced to Ixx*Ec/Es and then added to Ixx (girder).

Similar things are done for Iyy, area,Izz and other properties. We can see that area of deck is higher but the total are of deck + girder is less because the area of deck is reduced by Ec/Es and then added steel properties.