The measurement is triggered when the ball crosses the vertical line. Check if the vertical line on the measurement screen aligns well with the net. Move your tripod left/right and rotate it such that the alignment of the vertical line with the net is good.
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You need to align the smartphone camera with the net, not the tripod itself. The tripod will be a bit to the right of the net line.

If people are moving in the background or the players are visible in the measurement screen, the measurement is sometimes also triggered.