When is Delete a delete not Trash

I have been in many occasion troubled by the fact that Delete icon, or The word delete doesn't mean delete something. Rather it means move to trash. For example in Gmail that is the case. I was trying to help someone about their Gmail account not having much space, I saw Gmail was taking up like 14+ GB of space, so I told him to delete mails from Inbox. My bad I didn't realize that delete isn't delete, and they have to clear up Trash.

Friends would ask their computer doesn't have storage, yet they barely have something on it.

I wonder, on how may occasions people found themselves in trouble because the Delete button/icon isn't "deleting" things?