It's about custom field. Please read tech note for basic use of it. Frank, I think you should prevent indexu generate custom field form for your add.php and modify.php. Just delete tag <%custom_field_form%> in add_form.html and modify_form.html to do this. Then see the result.
You'll see indexu do not generate form field for custom fields. You must do it yourself manually in add_form.html and modify_form.html.
PHP Code:
<textarea cols="80" rows="20" name="your_custom_field_name">
<%your_custom_field_name%>
</textarea>