Cell Phone Vibrations VS. Ringtones; Why Does a Cell Phone Always Vibrate Before It Rings?

Have you ever noticed, that when I a cell phone is set to vibrate and ring at the same time, that it will vibrate before it rings?  It could be that the electric signal inside the phone is sent at the same time, but since the vibration is mechanical, it happens immediately, but the ring tone has to be accessed from a drive which has read-times that are slower than than the vibration.  What do you think?  It seems to happen on all models.