I'm using the helix\spiral to create a serpentine, but a question is arising when using the resource.

I'm using variable pitch to create a serpentine with different measurements. When I try to use the measurement to make a 1,500mm revolution, the measurement is different:

I understand that when the 1,500mm measurement is added to the distance from the end of the first revolution to the last, it should present the specified measurement, however, when detailing, it is noted that the measurement is different:

The measurement should be 1,500mm, but it's actually 800mm.

Could someone tell me why the measurements are different?