Yes, that must be it. On the ancestor there must be some code that adds the item number to the title. Something like this:
this.title = this.title + ':' + string(this.il_itemnum)
Or maybe it uses another instance variable where the original title is stored:
this.title = this.is_original_title + ':' + string(this.il_itemnum)
Check the code at the ancestor window to see if this is the case.
By the way, the behavior of the general tab of the window painter is correct. When you add an instance variable to an ancestor it will appear as a modifiable property in the general tab of the inherited window so you can set its initial value that'll be used when an instance of the window is created. Or it can be left as is and set through code at run time.