Categorization records not only which category items were put into, but also the order that the items were placed into the category. For each row, only one column should have a value. Which column that is indicates which category the item was put into. Which value is recorded indicates the position of the item within that category.
You could temporarily remove this from the question's CSS to see how the responses change in real time:
#[% QuestionName() %]_div > .question_body > .inner_table {
display: none;
}
If you placed the first three items in the first category and the last three in the second, I'd expect the data to look like this:
Categorization_r1_c1 = 1
Categorization_r2_c1 = 2
Categorization_r3_c1 = 3
Categorization_r4_c2 = 1
Categorization_r5_c2 = 2
Categorization_r6_c2 = 3