Why does the indentation of my numbered list change at 10?

I created a numbered list in InDesign CS6. The left indent is 0.1875 in and the first line indent is -0.875, so that all of the lines are aligned.

The numbered list looks fine until I reach 10. For 10, 11, 12, etc, the first line is indented an extra .25 in, but all the other lines are still indented 0.1875 in. The numbered list looks something like this:

1. sdfds
   sdfdsf
2. dsfsd
3. dsfsdf
4. dsfsdf
5. sdfsd
6. sdfdsf
7. dsfsdf
8. dsfsdf
9. sdfsd
10.     sdfd
    sdfdsf
11.     sdfasd
    sdfdsfsdf

Any ideas?

Thanks.