You are right. In speech a lot of Germans or Austrians just say geöffnet, it is more common, more natural. If you translate it literally, you see that "geöffnet" means "opened" and so you say "my door is opened", but that is just the way native German speakers say it. So: "Mein Tür ist geöffnet" = "My door is opened" and "Mein Tür ist offen" = "My door is open".
Not a native english speaker as well but from what i have understood i will try to explain. The door is open means that the door is open right now and it was not open before but it is open now but the door is opened is passive form and it refers to a real action in a certain way . for example the door is opened on nine o'clock on wednesday it is a real action and the door is always open on mentioned time please correct me if i am wrong danke schön
''Open'' is the past participle form, it implies that something has been opened, whereas ''open'' is simply the adjective to say that something is open :) http://forum.wordreference.com/threads/eyes-open-opened.2417697/ (the post is in Spanish)
Native English Speaker here. "The door is open" is in the present tense, or right now, so the door is still open. "The door is opened" is uncommon, but tends to mean the door was opened earlier and is still open. "The door was open" means that the door was opened, but is closed now.