A line delays starting at 9am?

I check Metro’s website every morning before my commute so I can be prepared for any delays. I saw that there are going to be 20 minute headways on the A Line between 10am-2pm, but by the time I got to the Pomona station at 9am the 20 min headways had already started. Are they using a specific station as their reference point? For example, if it takes an hour for a train to go from Pomona to downtown, it would make sense that even though the delays start at 9am at Pomona the delays wouldn’t start downtown until 10am.

How can I determine when the delays actually start at my stations? Just trying to plan my commute better!