A seam in the sail

Original non-fiction-style practice text.

The racing crew wanted their sail back by Friday. At the workshop, Mara replaced a torn panel and spread the sail across the floor. The new seam looked straight. A crew member photographed it and asked whether the repair was finished. Mara put the sail on a drying frame instead of folding it into its bag.

The cloth had arrived wet. While it remained damp, its weight pulled the panels down and disguised small differences in their shape. The thread along the new seam also needed to settle under tension. A neat line on the floor did not show whether the repaired area would carry a load evenly when the sail was raised. Mara wanted to examine the whole curve, not merely the strip she had stitched.

By Thursday morning, a slight ridge had appeared beside the replacement panel. She unpicked a short section and adjusted it. The crew member returned, saw the loose thread and thought the work had gone backwards. Mara showed him the ridge before finishing the seam again. Waiting had created no new fault; it had made an existing one possible to see. When he collected the sail on Friday, he took another photograph. This time he stood far enough away to include the frame.

Explain, using your own words, three reasons why Mara does not regard the straight seam on the floor as proof that the repair is finished.