The horizontal progress indicator on this page is funny (a blue strip on the bottom if you look). It looks like it's moving from left to right to meet the scrol

The horizontal progress indicator on this page is funny (a blue strip on the bottom if you look). It looks like it's moving from left to right to meet the scroll bar which is going from top to bottom. Several online publications have used this web design pattern and it confuses me. What's wrong with looking to the scroll bar position to know how much text is left on a page?