If you have a question you would like to have answered click here . The reason you want a more open choke is to compensate for differing ballistics of steel.
Steel tends to pattern more tightly than lead.
In other words, the grouping of lead pellets hitting a target at 25 yards will be more spread out and more likely to hit a moving target. Steel shot fired through the same gun with the same choke will hold a much tighter pattern. By going to a wider choke with steel shot you may increase the effective pattern of shot.