That is just how they decided to mix the sounds together. I'm not sure which font you are referring to, but if we are talking Sabercore 2.0 then the Veridian don't is the closest to the movies with the Crimson font being mixed to sound more "Darkside" and with a lower pitch.
If we are talking other sound boards then the hum or any sound is purely a product of how that font was mixed. Some fonts take sounds straight from the movies, other take those sounds and distort them for various effects and different "feels" to the sounds.
There are some really great sounds fonts available for both Plector and NEC boards that feature sounds direct from the movies.
There may also be some copyright issues if you were to mass produce something that used sounds directly from the movies.
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