Some cultures are made to do identical work but they have different strains of the same species. They have similar name - only the last digit is different. Some examples are MA4000, MA4001, MA4002, or MM100, MM101, MM103, TA50, TA51 to 56, TA60 to 62, LBC 81 to 82, MA11 to 19, etc.
It usually is no concern for the home cheesemaker, but creameries rotate cultures regularly to prevent phage: If something attacks a given strain of culture, when they switch to the next number in the series that phage will not know how to attack a totally new strain so it will die off and disappear. The cheese however will be exactly the same.
It's just like how Influenza get smart on us. We develop immunity to one strain and then it switches the strain on us and we get the flu, even though we got a flu shot, right? Next season the new shot will outsmart that influenza strain. In the case of cheese, the culture needs to behave like that smart influenza. Does that make sense?
Pre inoculating is a technique to propagate some lactic bacterium slowly in pasteurized milk, to grow back a few off the flavor elements that were lost during pasteurization. When you do that, you also create a competition of species which could have the benefit of defeating or weakening some pathogens.