The zoomstart event is only triggered for animated zooms.
For normal zooms there's no zoomstart. If there were, you'd get it at exactly the same time as the zoomend, because the entire zoom is processed in a single "chunk" of uninterruptable Javascript.
[Javascript events don't interrupt. Events gets queued until the "sender" of the event releases control. So if the API triggered a "zoomstart" then did all the hard work of recalculating the polylines, then triggered a "zoomend", both events would be queued until the entire process was complete. In the case of animated zooms, the API performs a setTimeout() between animation frames, which releases control.]
-- Mike Williams
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