You will probably find that the intermediate frequency of the radio is something like 10.7 mHz. If you double that and add it to the frequency you were tuned to you'll find it comes out pretty close to the frequency you were hearing. Little pocket radios - and many of the scanners so beloved by spotters - are of poor quality and lack "filtering" to prevent such problems. They are wide open to strong signals too. If a police car or taxi drives past you whilst the occupants are using their transmitters you'll probably hear them too, no matter where your radio is tuned.