Trained marksman would take a center of mass shot at this range to avoid the possibility of missing. Headshots are for Hollywood movies or in situations where a center of mass shot isn't possible. You simply have more mass to hit. 
This likely wasn't a trained marksman. 
Source: Ex US Army sniper (me)
Your assuming he had a clear shot to center of mass. For starters, likely that Trump was wearing a good vest. And I'm sure they put bulletproof plates in his podium.
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