On vintage guitars such as Gibson archtops and Fender Stratocasters, there would have been no difference at all.
Eventually, somebody realised that winding the bridge position pickup a little hotter than the neck position evened out the differences in output due to different degrees of string excursion above the two pickup positions.
On a modern calibrated pickup set, the bridge/treble pickup can afford to be a hotter wind and to put out more mid and low frequencies. The neck/rhythm pickup would be wound for clarity.
Having said all of this, there are no absolute hard and fast rules. I, for instance, use a SP90-3n as the bridge/treble pickup in a Les Paul.