I use two apps on my Android device for calls. One is True Phone which has replaced my default dialer. Even though the default is very good, I have a preference for True Phone. The next one is Callistics. It keeps an independent log of the calls / SMS / data for logging purposes (so you know if the phone company trying to cheat you).
If you had Callistics installed you would see the number in that list even if you removed it from your phone. Just saying. In case you get upset and want to delete her again.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".