For whatever reason, the center tap of the transformer has a series inductance equal to one quarter the primary inductance (the primary winding being the center tapped winding). On the left is a 1:1 center-tapped transformer model with an inductance of 10H and an extra inductance of 2.5H in series with the center tap and on the right is a 1:1 EveryCircuit™ center-tapped transformer of 10H. As you can see, both behave identically showing the the EC transformer has a hidden 2.5H inductor in series with the center tap.
