Normally it takes 4-5 hours before ppl start passing stones and not 2 hours. ( If we drink that yucky oil+juice concoction at 10:00 PM, earliest I have seen stones arnd 3:00 AM)
There is complex chemistry going on inside the body. The oil, the bile, digestive enzymes etc. can all act together and bingo, you got these so called stones. I have read stories of ppl who claim they are stone free( 3 clean flushes in a row) and still have problems.
For ppl, who have produced ZERO stone, it could be possible that there was not enough bile or 'right conditions' to produce those stones, and they got the impression that their liver is cleansed. But if they keep going they WILL produce more stones. Here is an example of an ardent flusher