I have also run into this in Word 2013.
An EASIER FIX is to go to your VIEW tab on the top bar.
You will see the following subcategories within the VIEW tab: Views, Show, Zoom, Window, and Macro.
Under the "Show" category, put a check mark/select "Ruler"
A ruler will appear along the top and sides of your document.
You will see an "L" shape in the top left corner of the document. A little farther to the right, there will be an triangle pointing down, a triangle pointing up, and a rectangle. They are usually together.
Now: Highlight all your items within your numbered list. Make your view so you can see a few numbers under 100, and a few over 100.
Rest your cursor on the up triangle, and move it over slightly to the right. Watch your margins of your numbered items as you do so.
All of them will move; the ones under 100 will move to the right, and the ones over will go to the left.
What you have done is increase the area for the number, so that the text of your numbered item doesn't go to the next tab stop.
Hope this helps. :)