Station Fixation has an article about this. From the article:
"At Christie, the escalator was a retrofit, installed well after the construction of the station. The device’s base is located above the track ceiling below. There physically isn’t enough room for the mechanical equipment at the base of the escalator, for it to extend all the way down to the floor of the mezzanine level.
Similar escalators with steps may be found at Broadview and at King."
Photos and description of the TTC's Christie subway station.
www.stationfixation.com
Transit Toronto also mentions the escalator:
"One of the odder features of Christie station is the escalator leading down from the main entrance to the mezzanine level. The escalator bottoms out a couple of feet above the mezzanine level floor, and passengers have to proceed down a short flight of four steps. This is because this escalator was added after the station opened. At this point, the escalator is directly above the subway tracks, and there is not enough space between the mezzanine floor and the platform ceiling to fit the mechanical workings of the escalator at its base."