Let’s say you have a set-type attribute $Colors. And let’s assume that within your file there are notes that have values blue, red, green, orange, and so on.
To the best of my understanding, the drop-down list is set up to show you all the values that exist for that attribute. So if one or more of your notes have a value of orange, orange is going to be among the values you see. (On the other hand, if you delete that note, or remove its and any other note’s assignment to orange, orange will no longer appear on the drop-down list – unless you have pre-populated it.)
So the list is (as I understand it) designed explicitly to show you all the range of values for a certain attribute. To get a truncated list, you’d need to set up some other attribute and make sure that you assigned it only the values you wanted to see on a list.