The Physics behind this is simple - strings (and almost all magnetic pickups) are "flat" at low frequencies. Doesn't matter the construction, there is nothing that takes away (or adds) low end from either. Low mids - pretty much the same thing. When it comes to upper mids and treble content, though, there are things going on that make different strings and pickups work differently. So, why do certain strings or pickups sound bass shy? It's because they're screaming at you at higher frequencies, and that dominates your perception of them - too much treble sounds like there is less bass, but...there isn't.
When you put a pickup at a different spot, now there are things that change the amount of low end, but it isn't the pickup - it's the position.
When you put a pickup at a different spot, now there are things that change the amount of low end, but it isn't the pickup - it's the position.