X-Plane only supports whole integer frequencies for ADFs, hence I assume that limitation is enforced by the avionics so as to now give a false impression. Therefore the navigation aid data provided to be used by X-Plane forces the PA NDB, in this example, to 307 kHz. I assume the rounding down is consistent for other NDBs, but I haven't verified that. You could report this frequency limitation to Laminar as a "bug" in X-Plane, to be honest. (They are quite US-centric so might not realize it's an issue.)