They have several different humidity levels.
I was responding to the OP's question about why the 75% Calibration kit instead of a regular basic Boveda, and Joe45's hypothesis that the calibrating to 75% made the hygrometer more accurate at other levels.
What I meant was that the old "salt test" calibrates to 75%. So when manufacturers made the push-button calibration hygrometers, they made 75% the number the hygrometers reset to, since the salt test was generally the only way to calibrate at home back then.
So when Boveda put out their calibration packets, they made the test bag a 75% bag because that's what all push-button calibration hygrometers reset to. It has nothing to do with accuracy at other levels, it's just what the salt test gave and the rest followed.
In other words, there's nothing special about 75%. Unless you're using a "one-button calibration" hygrometer, use whichever bag you like. In fact, use the closest to where you want your rh to be, that'll give you the most accuracy.
My process is to use the 75% kit to measure, reset the hygrometer to 75%, wait 24hrs to ensure stability, then transfer the hygrometer to another bag with a 65% Boveda, wait another 24hrs and confirm accuracy again. So far it's worked out for them within +/-1%