Yes this is true... Nearly all modern Ibanez have a high pass filters hard wired to their volume pot... the JS
is the only one where you can choose to disable this ... have a look through the diagrams here... the "331" cap on the volume pot in each diagram is a high pass filter... only the JS
has this on a push pull....
all pots of the same resistance value and same taper are the same ...
The easiest way to describe it is to try and not think of the signal comming from your pickup as one signal, but as hundreds of thousands of signals all at different frequencies travelling all at the same time.
These signals react differently when they hit the 500k ohm barrier between them and the gound that the pot creats... The high frequencies find it much easier to get though this barrier then the low frequencies and hence the effect that you get is that you lose the high frequency .. and as you lower the resistance and the jump to ground gets easier, more high fequencies escape so that the sound gets muddier and muddier..
When you bypass the pot with a capacitor for all intents and purposes you reverse this effect... what happens is that capacitor acts as a road block for low frequency sounds but lets high frequency sound pass through relativly easy... When the pot is at full value it makes little difference as its easy for all the differnet frequency sounds to escape out to your amp... As you turn the pot down and the resistance drops some of the frequencies start escaping to the ground but the high frequencies find it easier to go through the cap so continue to escape to your amp.. wuntil finally its easier for them to go to ground then go through the cap and your sound dissappears.
this is an overly simplified description whose attempt is to describe the effect rather than the means