Dropdown Range Changes When Re-opening File?

I have a list of expenses on a certain project. I allocate each line to a certain type of expense by using a dropdown list. When re-opening the file, the first few lines were good: dropdownOK.jpg

But after a few lines, the dropdown range changed, although it keeps the selection I made in the previous version: dropdown_wrong.jpg

I have seen that the data validation list changed to =$A$2:$A$17 instead of my separate list =Dropdown!$A$2:$A$17 but cannot understand why excel has made this twist. Each month I update this list, which means adding lines to the list, both inserting them at the bottom, as in between other lines. I can't see any pattern, because it kept the range of the first 6 lines, when the previous version already had 50 lines or so. It does not only happen to the inserted lines, for example.