When you simulate the antenna with a 50 Ohm port without the capacitance you'll get the impedance of the antenna for example if you look on the smith chart.
But the S11(to the RFID Chip) and the accepted power of the antenna are wrong since CST uses the 50 Ohm port. You have to calculate the values by yourself.
When you insert the capacitance, the imaginary part of the simulated input impedance should be zero. The imaginary part of the antenna impedance and the impedance of the capacitor are summed up to zero, which gives you a zero imaginary part seen from the lumped port.
And if you use a port with 25.5 Ohm the S11 should be correct.