Whistles have to be designed or adjusted - depending on the pressure used to blow them. The slit in the bowl the steam blows through has to line up with the bell, and the distance between the bowl and the edge of the bell varies with pressure. Most smaller generic single chime whistles are specifically designed to adjust this by turning it on the threads of the shaft.

Blowing a steam whistle on air also changes the tone, as will impuities in the steam. Castings were not always exact, so the same used whistles of the same design will sound different due to the steam, casting and machining tolerances, and wear.