Mold is a living thing that grows and throws spores all over the place. It’s literally everywhere, all the time. The non-moldy bread has mold on it, too, you just can’t see it, because it hasn’t grown big enough.
Some of those spores land in the right places to grow very fast and some don’t. Some loaves get more spores than others. Some loaves are in tightly sealed bags and some aren’t. Some loaves were packed on the day that the machine was cleaned and some weren’t.
It really is as simple as that.
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