Why all the re-rendering of already rendered renderings :)

I have this strange FCPX behavior: when I'm in a project and everything is rendered, when closing down and opening it again, some parts will begin rendering again......... when they're finished, I can close down and open again, and OTHER parts will then need to be rendered. Is this normal?

Are you manually rendering, or relying on background rendering?

I'd turn off background rendering, and only manually render as necessary, which is not that often,

Automatically - I like that feature.

It should only do that if the media has changed in some way. Is your media stored in multiple locations? Could it be that optimized media is available sometimes and not available at other times?