Because in the real world there is pipework that wasn't installed today and that has a biofilm and storage reservoirs  have to be vented thereby allowing air to come in and out of the tank and this can carry contaminants even if there is insect screening.

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Because in the real world there is pipework that wasn't installed today and that has a biofilm and storage reservoirs  have to be vented thereby allowing air to come in and out of the tank and this can carry contaminants even if there is insect screening.