centre tapped transformer calculation
I assume, you measure the voltage with an oscilloscope (I know no other instrument that is measuring regularily Vpp). Why don't you just try to understand what you see instead of comparing assumed values with measured values? If you don't have an oscilloscope, it is actually difficult to understand the circuit behaviour exactly.
Basically, the calculation would be correct for an ideal tranformer, as I already mentioned. Assuming your winding ratio specification is correct, the measurement shows, that the transformer isn't ideal, most likely the difference is due to stray inductance. What causes the difference can better be seen with an oscilloscope than predicted theoretically, although it can be calculated and simulated if necessary.