Hi Cristina,
The reason for this sorting is because the merged column would be a character field. Thus the sorting would be a combination of the number and text value.
If you require the sorting to be based purely on the ascending order of the number, create a duplicate dimension with only the numeric field (if not present already). When you sort the numeric dimension, your merged column will rearrange automatically.
Please let me know if there are any questions or comments.
Thank you,
Varun Anand