Index formating Issues

We have a numerical index being generated, but the numbers are in sequential order sort of. We have a mixture of 4, 5, 6 digit part numbers the 6 digit numbers are listed first in sequential order, then it displays the 5 digit numbers in sequential order, then it displays the 4 digit numbers in sequential order, then it does the few alphabetic entries that are in it. Where do you change it, or can you even change it? In styler I have found the indexes tab, and from there checked out the formating tab. I did not see anywhere to make these adjustments. I also found sfe:indexAlphaGroup_Main_Index, also did not see anywhere to make these adjsutments.