The 550 capacitors for Humbuckers are supposed to make the sound more clear but for some reason the 500 is the average.
That is also a lot about the tolerance. Normal pots usually have a tolerance of 10%+- so when you buy a 500k pot there is a chance youŽll get a 550k or a 450k pot. These divergences in value make a noticable difference in sound and 100k possible difference is a lot. Anyone who has tried running a HB on SC valued pots, 250k, knows this all too well.
Thing with the BKP pots is that they are on the high end of the "normal" spectrum with very low tolerance, so you can be pretty sure you get what you pay for.
The slightly higher value does a similar thing to what scatterwinding does, adding that extra highend, overtones, etc, but in the case pots it is more about letting them get out to the amp. Nothing you usually wanne overdo, but getting these BKP pots ensures that you get good quality, low tolerance pots, which make sure that your PUs are not inhibited by them. Not to say that you canŽt get this elsewhere, but ususally it is more of a gamble due to tolerance.