Xperia X8 :: It Does Not Always Play Contact's Custom Ringtone

I set my wife's contact with a custom ringtone. Sometimes when she calls, the phone plays the custom ringtone. Sometimes she calls, and it plays the default ringtone.

I check the settings, and the custom tone is still set correctly. And the ringtone is on the phone, not on the memory card.

Today I noticed that when this happened, the phone started ringing the default tone while her cell number appeared on the screen, before the phone detected it was the contact with her name. Could it be that the phone delayed recognizing her number as being on the contact list, and therefore played the default tone?