[Goldeye]

I am guessing that it has something to do with Peukert's Law. http://en.wikipedia.org/wiki/Peukert%27s_Law
If I am interpreting correctly, Peukert's Law says that a battery's capacity (units: charge) decreases when it is being run at higher currents.
The battery does output a lower voltage as its remaining capacity dwindles, so perhaps Peukert's Law implies that while the current is high, the battery 'feels' like it has less charge remaining than it really does and can only output the voltage it would if it did in fact have that much charge left.

http://autos.groups.yahoo.com/group/.../message/43491, asks if saying "expressing Peukert's would be an expression that at "x"%
state-of-charge the battery can sustain "y" amps without dropping below
10.5v " is appropriate (for a car battery).

So if Peukurt's Law is indeed the cause, squirrel's explanation is, although a condensed version of the story, correct. Elsewhere in that yahoo thread, someone explains it as "What Peukert does is try to model the cumulative effects of polarization, electrolyte starvation, depletion of active material on
the surface of the paste(s), ohmic loss and probably a few other
things."