Continuity mistake: When Luther Billis is parachuting out of the plane, he is seen with a pistol in his hands. Once he is in the rubber boat and starts paddling, however, the pistol has become a rifle.
Continuity mistake: When doing the "there is nothing like a dame" dance number on the beach, Ray Walston from cut to cut has his shirt either buttoned up or unbuttoned.
Answer: Richard Rogers preferred the vocal performance of Muriel Smith who played the part in London. Since the vocal tracks for musicals was recorded separately from the filming back then, dubbing occurred for many reasons other than lack of singing ability of an actor, scheduling problems, budget, etc.
Myridon