Interesting question. James Willett, who equalled the WR 2 years ago, does the same as MD- fixed full top and removeable bottom, extended tube in his case. I don't know what he uses in terms of constriction.Why do some international trap guys shoot a choke tube for the first barrel? I also noticed this on a few ISSF videos so it must not be rare. I understand that every target counts but I would think it wouldn’t make a difference for the world’s very best.
Some reasons may be if they decide to shoot an #8 in the bottom, they will usually go one constriction (i.e. 4 thou) tighter. This may depend on ammo availability, for example-you can't fly with more than 5 boxes, and if you usually shoot an #8 but the competition in Egypt or wherever only has #7 1/2, you may go from an 8 choke to a 7 in the first barrel.
It may depend on target hardness. I have spoken to one international representative who said at one competition the targets would move 1 metre sideways and not break. So in that case they may "choke up".
Or it may be that they use say, a 7 in the bottom for qualifying (where 2 shots are allowed) but want to change for the finals or a shoot-off where it's only one shot.