Why do some kids with chromosome problems have a webbed neck?

Misinformation. The chromosomes are the library where the body/tissue gets information on how to develop, form organs or structures. When there is extra or missing chromosomal material, part of the information, or assembly instructions are gone or confusing. The body parts on a patient with a chromosome defect will have defects in organs/structure or appearance based on this misinformation.
Webbed neck. Webbed neck is associated most commonly with two different conditions, turners syndrome and noonan syndrome. Only turners syndrome is associated with a chromosomal abnormality. The chromosome number is 45( instead of 46) with only one sex chromosome (x)= 45, x.