in addition to the tabstops that you specify, mIRC continues to assume additional tabulated columns be 5 spaces apart, past the last tabstop you specify.
Since you specified a tabstop of 2, and 'abc' is 3 characters, you already exceeded your first (and only) specified tabstop. The next tabstop lands on 7 (2 + 5), and the next wil be 13.
-V----v----v----v
abc cde
Try specifing tabstops further apart, or data that actually fits in the column widths you specified.
- Raccoon