it's because you have a siren and with a siren, the sound will fluctuate up and down (hence the name siren). So when you arm/disarm your car and depending on it the sound is on the rise or on the low, the sound will be different. In your case, the sound also cycles over and over again and you might of caught it when it's about to cycle over again, that's why you are getting a half chirp.
I have the same issue with my siren. Like people have stated earlier, if you want a consistent sound, you might be better off getting a Piezo buzzer.