Yes, Ben, I think you might be on to something. The amount of residual warmth from the strong El Nino is very substantial. Both north and south of the immediate equator, positive SST anomalies remain, and may be making it hard for the negative anomalies right along the equator to "take over". A very narrow band of negative anomalies presents a weaker trigger for the onset of enhanced trade winds than would a latitudinally wider band. So your point is a good one.