Do rims improve with age?

[grandaddy]

I've just got a new PD-105 and I've noticed the rim is less sensitive than my older one...similarly I checked my old PD-80R against my much newer PD-85 and the difference was significant.

I've also read the the bell sensitivity on the CY-15 seems to improve as it gets older...

Is this a documented phenomenon with the dual piezo pads....I'd like to know if others have experienced this or is it just some inconsistency from pad to pad?