Import form data via txt file

I have a PDF form which has a combo box and a list box.

The combo box has options 1,2,3,4. And the combo box has options Male and Female.

I have exported the form data using the .txt format.

Then I get a txt file with the choosen options. Emmm, it is quite right.

Then I re-edit the txt file. I have changed the value in it. And then I import it to the same PDF form.

A intersting thing happend. There is no change happened for the combo box. The new value for the combo box hasn't been imported at all.

I have seen the value.

Why?