fully-differential amp measurement using 4port SP

For fully-differential AMP, I want to measure some of the most important paramters such as differential voltage gain, input impedance, output impedance.
I have tried two method: one is to measure 4 port Scatter-paramter using network analyzer, and i got .s4p file and Sdd21. And in order to validate it i mearsure the 2-port SP using the configuration of Balun(Single to Double,T=0.5)+DUT+Balun(D to S, T=2) and i got the s21. In my foregoing deduction, the s21 and sdd21 should be same, but the measured data is so different, sdd21 is about 10dB greater than s21!
WHY? I am confusd.

The two measures should match.
Check balun parameters. For example, if your DUT has single end impedances equal to 50 ohm, you should use a single to differential balun with 1:2 impedance ratio (if you're using ADS take care that the T parameter means turn ratio and not impedance ratio, being Zratio=sqrt(Tratio)), DUT and balun 2:1 impedance ratio.
Check also S11 and S22 and compare it with Sd11 and Sd22 respectively.
I hope it can help.
Mazz