What's happening:
Applying Numbering automatically includes an indent after the number. Unless
you consciously set some tabs, the tabs default at every .5 inches. For the
first 99 numbers, the one tab jumped to .5 in. But 3 digits pushed the
number just past .5, and so one tab had to jump to 1.0 inch.
How to fix:
Um, good question. Assuming you applied Format | Bullets and Numbering in
the first place, go back there, click Customize and change the "tab after"
settings. OR, change the number position from Left to Right, so that the
numbers align at their end instead of their beginning. I think you need to
select all the numbered text first, and make sure to use the same numbering
option as before.
Better fix:
Use Format | Bullets and Numbering to remove the numbering, and instead of
using direct formatting via that dialog, apply a numbered style to get the
numbers, like say List 4 or List 5, which have a deeper indent built in, but
may still need to be modified to align the numbers differently. Why?
Because in general Word will be less frustrating when you use style-based
formatting instead of direct formatting.
http://www.microsoft.com/office/previous/xp/columns/column14.asp
Scroll down to the Qs "how do I modify..." and "what is the advantage..."
