For them, you are a random event. Back in 1905 a Danish telephone engineer named A. K. Erlang studied the predictability of such things. Among other things, he even devised a formula that could be used to calculate the expected waiting time on a given line. Accepting lines and waiting as inevitable, traditional manufacturing depends on Erlang's brainchild and its successors in queue theory. And it's true that in a traditional approach there is plenty of randomness.
Customer behavior is often random.
Parts and supplies arrive in a random fashion. Randomness also creeps into production. The result, as we all know, is frequent fire-fighting, a certain amount of yelling and screaming and the inevitable waste of time and energy. But lines aren't inevitable. Lean manufacturing's pull system approach is essentially a way to eliminate as much randomness as possible.