At what frequency is a transformers rated impedance measured
If you look at the data sheet it does indeed say that the impedance is 50 ohms across that range of frequencies however, on closer inspection, it gives a return loss figure that does vary a bit and is at its lowest at 900 MHz with a value of 18 dB.

Having said that, 18 dB is a pretty reasonable return loss and implies that the 50 ohms (perfect value) might be about +/- 27% different at 900 MHz. At 100 MHz the return loss is about 29 dB implying an impedance accuracy of +/- 7.4%.



Return loss = 20\$log_{10}\bigg[\dfrac{Z-50}{Z+50}\bigg]\$