Why is my spline tool deforming my wall?

Hello, I am using a tutorial to learn how to spline walls. In the video, the instructor is able to stretch the spline out and the wall itself doesn’t visibly stretch or deform. However, it is doing so for me, and I believe this is causing me an issue further down the line. Can someone explain why this is happening and how I can resolve it? See screenshots they should illustrate the problem.