A cumulative probability function specifies how likely the next reinforcement is to have occurred as a function of how far one looks into the future.
In a random rate process, the probability that the event will occur in the next instant is independent of how long it has been since the last event. Thus, as time advances, the red function advances along with it. The subject, so to speak, never catches up and passes the function. The green function, by contrast is the function immediately after the onset of a CS that predicts the occurrence of the US at a fixed moment in the future. This function is anchored in time; as the subject advances in time, it catches up with and passes this function.