Another reason is choke selection. With fixed choke guns (yes, there are still guns made without screw-in chokes) the lower barrel's choke is typically the less constrictive of the two. When wing, or target, shooting it is assumed the target will be going away from you, thus a tighter choke is desirable for the second shot; therefore, the first shot (or singles) is taken with the lower barrel's wider choke.
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