Between the chapter number and the chapter text, there is a tab character. The text "Chapter 10" is longer than "Chapter 9". This one additional character is pushing the phrase past the original tab stop so it going to the next one.

Modify the style to set a tab stop. It is difficult for me to see in the graphic, and I suspect your ruler isn't showing inches like mine does, but try setting the tab stop at about 3.5.