Interesting question. The word "off" here is a preposition. It means "from" as in "to be separated or removed from something else". This is the literal meaning. The literal interpretation is that the idea or explanation which you know very well, sits at the top of your head. If you don't know the concept very well, then it is not on top of you head so then you can't get it "off the top of your head". This phrase is idiomatic and should not be interpreted literally. English speakers are definitely not thinking about any literal meanings when they hear or utter this phrase. Therefore the most useful interpretation of this phrase is that you cannot provide the information sought without some time to think or other aids like a book or the internet, which ties in with the "not immediately" connotation as in the answer given by Altini.

Not immediately. Not right now.

It's hard to separate the meaning of "off" in this set phrase. Something is "off the top of your head" if you don't need to think very much about it. So to say "not off the top of my head" is basically saying that you can't come up with an answer quickly. Maybe you need to look it up, or think about the question for a while, or you need some time to remember something. Here's a link to a possible origin of the expression that might be useful. http://rollsoffthetongue.tumblr.com/post/98501134420/off-the-top-of-my-head-origin-mid-20th-century

The idea behind this expression is that you can or cannot simply take off or remove a spontaneous idea from your head. Off is close in meaning to "from" here.