Q: Why does FCP X re-render projects that have already been rendered?

I've seen several posts about this problem, but have not seen a solution/explanation yet.  I have a project in Final Cut Pro X that I've finished editing.  I have background rendering on and I wait until the entire project is fully rendered.  I then shut down FCP.  When I restart, it reloads the project and then begins re-rendering it.  The whole thing.  This happens every time and the process takes quite a while (even though it's a short video.)  I've trashed my preference file, but still it does the same thing.  I can't even begin to tell you how much grief this has caused me.  So much so that I just used the word grief for one of the very few times in my life.  Honestly, I just bought this MacBook Pro (after years of struggling with Windows) in hopes that this would be better than a PC.  That doesn't seem to be the case.

Thanks for any help you can give me.