By fundamental I mean the fundamental frequency of a note, also called the first harmonic. Low end is generally considered the fundamental and lower harmonics (multiples of the fundamental frequency), highs are the upper harmonics, midrange is what's in between.
The Gibson mudbucker was designed for "pipe organ lows", as Bill Lawrence put it, without much regard for mids and highs. The closer a pickup is to the bridge, the less the string movement over the pickup, and as a result, it doesn't pick up as many lower frequencies, which is why any pickup sounds brighter in the bridge position. With the Gibson, its power comes from those lows, so in the bridge position it won't be as strong sounding because its mids and highs aren't that strong. Hope that makes some sense.
Sometime you might also want to look up Fletcher-Munson curves.
