Money is very dirty...a combination of anything and everything that has ever touched it, sometimes including faint traces of drugs! Sooooo...as you dig money out to give to the dealer in exchange for chips, the money-grunge transfers to your fingers. From your fingers, it transfers to the chips and becomes chip-grunge.
The extra grungy spots are a result of drinks. Either a drink splashing on the chips, which would leave a sticky spot that would attract extra money-grunge...or a money-grunge covered hand picking up a sweaty drink, thus liquifying the money-grunge before it transfers to the chip.
Do people who are fanatic about cleaning chips also clean their money? Would that be called 'money-laundering'? 
