There are many times where you might find you purchase or have a large quantity of the same audio buzzer, indicator, or transducer that sounds different. The reason is because of the tolerance within the frequency range of the audio component. Many suppliers have ± ranges that can be even 500 Hz. This specification is called out on most data sheets so make sure to take note on design in. Usually when production takes place they are relatively close but if you purchase in future or at different times or lot codes you could go from one range of the tolerance to the other which means it will sound like you bought or have a whole different or incorrect unit. Many suppliers will offer custom frequency testing/sorting if needed for a customer but at a little different MOQ and price.