Why is Add or Remove available on create object activity but not change object?

I noticed that I can Add or Remove an object(s) for a reference set in a create object activity but NOT for a change object (in 10.4.1 and 10.2.0) for the exact same association.  Was it always this way? Wouldn't it make more sense to be the other way around? Or am I just not understanding what's going on here.   From MX documentation: "For a reference set association, it is also possible to add and remove an association (instead of only setting the member). For add, an object or a list of objects can be added to the currently associated objects. For remove, an object or a list of objects can be removed from the currently associated objects."    
1 answers

It is somekind of bugs of Mendix 10.

The 'Set' option is default and can't choose other options at first.

But after you enter the any value and click the okay button and edit it again, you can find that you can choose other options!


I hope my bad english is enough to explain how to figure it out.