Hi Kabil,
I dont think, its ALV error. It's actually your Internal Table contents are stored in that way.
It seems you have used some FM to explode the BOM to find its compoment. First row in your Internal table is BOM header and 2nd row onwards, its all the details / components of the BOM..
play with your Internal table contents to get the needed output.
Instead of single level ALV, you can try for Multi Level ALV to show the BOM & its components in correct format
- harshad