Because

shortcut1 := $a ctrl.
shortcut2 := $a ctrl shift

created different shortcuts. 

Why modifiers change receiver state instead of create new instance?

And it is fully unexpected that I can not refactor it to:

shotcut2 := shortcut1 shift

Because after that my shortcut1 become broken.

It was happen in TxTextMorph where selection and movement shortcuts created at same time

TxTextMorph>>moveEditorOn: aShortcut by: aSelector
   
    self on: aShortcut do: [
        self moveEditorBy: aSelector withSelection: false ].
    self on: selectionModeModifier + aShortcut  do: [
        self moveEditorBy: aSelector withSelection: true].

To fix it I add deep copy of given shortcut before modifier operation.

>
> There is another example:
>
> shortcut := $a ctrl.
> self assert: shortcut == (KMModifier shift + shortcut)
>
> Yes. Such operation modified argument of message #+. What the crapp! Why it not returns new instance?
>
> I hope you agree to change this behaviour
>
>