 Why did Callahan think Trillium was Berta when he first spotted her?
A dozen burners on this ship; why did something crazy have to happen to O'Rielly's? Well, ma'am, O'Rielly searched every cranny where even a three-tailed mouse of Venus could have stowed away. Somehow he doubted that Burner Chief Terrence Callahan's mood, or Captain Millicent Hatwoody's, would be improved by knowledge of she who was in O'Rielly's shower now. "Oh, I'm Trillium," she assured Callahan sweetly. O'Rielly stood looking thoughtfully at lovely, womanly, exquisite Trillium. Just like that, O'Rielly felt as sparkling of mind as a spiral nebula. Trillium—with her shape—passing as a boy hustling bags through this ship. "If what old woman finds whom?" a voice like thin ice crackling wanted to know.
 Because Berta was Trillium's Grandmamma and she resembled her from a hundred and twenty-five years ago.
 Because Berta was Trillium's Grandmamma and she resembled her from a hundred and twenty-five years ago.  Because she introduced herself as so and led him to believe that was who she was.  Because all the Venus women have the same enchanting appearance.  Because only Berta was able to enter the ship.