Hi n Jen / Raj,
Generally the attribute view column has the same data type as defined in the table. You cannot define the data type of the attribute in the attribute view unless it is a calculated column.
Can you please provide a screenshot of your KNA1 attribute view with the property window for KUNNR. Also please provide the screenshot of KNA1 table structure, if possible. It will help understand why the data type has been selected as decimal.
Regards,
Ravi