Sniper units are often deployed behind lines and spend hours or days in observation. They may arrive on site well in advance of their target and be very sure of

Sniper units are often deployed behind lines and spend hours or days in observation. They may arrive on site well in advance of their target and be very sure of identity before firing. Also, a marksman deployed with an infantry unit will support small arms fire (~200 yards) with range extension out to say 800 yards. A dedicated sniper team could be taking targets out as far as 3,000 yards. So perhaps a infantry marksman would be more willing to take a shot at a closer target given they are part of a squad, where a sniper team won't shoot at a tin cup or fake head in case they give away their position. It's worth just waiting another hour and see if that guy patrolling comes by again etc.