Your drop down field is based on the wrong field. The field used to select the category cannot be the same field as that used to identify the category for each record in your articles table.
If you have this relationship:
LayoutTable::Category = Articles::Category
Then LayoutTable::Category is the field to put at the top of your layout formatted with a drop down list for selecting the category.
If LayoutTable and Articles are two Table Occurrences with the same Data Source table, then you must use a different field for category in this relationship::
Table::gSelectcategory = Table 2::Category
Often, a field used like this for the drop down list is given global storage in field options so that different users can select values in the drop downlist without their choices affecting other users.