A timebase on a CRO needs a setup time before it triggers, so the signal comes in the trigger fires and say 10 NS later the time base runs. So if the bit of the signal you are interested in occurs before this time, you will not see it. To overcome this problem, the Y signal is delayed by a delay line in good CROs that delays the actual deflection, so if its delay was 20 NS, you can see a signal 10 NS before the trigger point. The other thing is the trigger level, the higher it is the more the display moves to to left, as the signal needs more time to get to the trigger level.
Look at the "cal" output, you should be able to see its rising edge.
Frank