Being somewhat generous, I don't think it's because they didn't understand the probabilities, it's because they didn't think about the impact of what they were doing at all, beyond the problem they needed to solve.
I.e., the only thing they were considering was whether the number entered at the start of the test would show up in the right places at the other end of their system. They simply didn't consider that an actual phone would be shut off along the way.
I.e., the only thing they were considering was whether the number entered at the start of the test would show up in the right places at the other end of their system. They simply didn't consider that an actual phone would be shut off along the way.