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.
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.