It depends on what brand, what guitar they are for etc. But in a vintage stratocaster, the bridge pickup isn't wired to a tone pot, so to compensate, they might be wound with a lower ohm resistance. Maybe 5.50k ohms. Where your neck and middle might be wound at 6.50k ohms. but it's not always the case. I have a set of 57 reissue singlecoil stratocaster pickups, all 3 of them read 5.47~5.50k omhs of resistance. For a singlecoil pickup in the middle position might be reverse wound to give it a hummbucking effect. Any change in the pickups from bridge/middle/neck can be done in the wiring also. So some companies might make the pickups different, or some might just make them all the same, and add specific components to the circuitry for the same difference.