As suspected, it's exactly a probability game. If there's few infectious particles sure to low concentrations (e.g. people distance or are outdoors), you would roll the 70% penetration chance fewer times, and result in very few particles reaching deeper into lungs too as the velocities are reduced.

When very close to an active particle source, you'd roll the conditional probability dice many more times.

The critical number is the base infectivity or viability. (CFU essentially) It is expected that sub-viable numbers might even work as a kind of vaccination if people isolate long enough. (stay at home for prolonged time without contact - scale of days at least)