Anyway, string tension isn't magic. For a given frequency and scale length, it's determined by the weight of the string. There's an exact formula for it: T=(UW x (2 x L x F)²) / 386.4 where T is tension, UW is weight in pounds per linear inch, L is scale length in inches and F is frequency. (h/t D'addario for the formula).
Since the core of a steel string is solid and the windings aren't, if you have two strings of the same diameter and one has more tension than the other, it's usually because of different core weight/size.
I'm talking about actual tension on the neck, not how stiff the strings feel.