When dealing with population biology/ecology you have to assume variation exists. This is even assumed when dealing clones with the EXACT SAME GENETICS because the cells don't behave like clocks on the same rhythm with every other cell in the population. However that being said mutations are common events that can lead to disadvantageous, adventitious, or neutral changes in the mutated cell. More often than not the mutations are disadventagous to the cell - where it will die or be likely outpopulated by healthy clones. These mutations can arise from any number of cases from DNA damage to a mistake by a DNA Polymerase when replicating the genome. So with that being said breweries order from these large labs where the and/or the brewery do constant Quality Control checks of the yeast. If the yeast comes from bad stock that is say for example ethanol intolerant at low levels of ethanol (4%), then the brewery wouldn't use the parent batch at all or they wouldn't sell that beer. That's my thoughts on it though - I have never worked in a brewery, but I do plenty of lab work and have seen other industrial applications of food microbiology.