Yeah, I think you already know what I mean, but just to clarify and how to reproduce easily. If you have 7zip installed (or I think any program that adds a similar context menu):
1. select a few files with Insert
2. press the context menu button (to the left of the right Ctrl)
3. navigate to 7zip and add the files to an archive
All the Insert-selected files and the currently focused-selected file will be added. If I enable checkboxes in the settings, I can see that the last file gets checked when pressing the context menu button.
When using copy/delete it's different, the last file (the not Insert-selected one) is properly ignored.
So, in short: the context menu creates a slightly unexpected behavior.