Hi,
I'm sorry but it is not clear.
Please provide precise information.
Did you configure the "ordering column" of your menu item to "product_code" ?
Or are you using filters to sort the products ? In that case, how did you configure the filter ? Could you provide a screenshot of its settings ?
Also note that if you try to order the product listing based on the product_code column, it's normal that 100 is smaller than 99 since the product_code column is a VARCHAR (meaning that you can enter letters, not only numbers) and thus the sorting is done alphabetically, letter by letter, and first letter of 99, 9, is higher than the first letter of 100, 1.
You would have to change the type of that column to "integer" instead in the hikashop_product via your phpmyadmin if you want to have an ordering done as numbers and not as text.