It doesn't in this font? Possibly because of what they talk about where it optimistically gives as much space as possible to succeeding glyphs. You can try it on the page; the code sample boxes are editable. I see no jumping when I type "calming", their example word.
[append] Oh, I do see some jumping when I type "optimized". Honestly, it doesn't seem very jarring to me; the jumping is always right where the edit point is so the added or removed letter causes a much bigger jump.