Another reason for bounces to be different from working projects ...

I believe this has been brought up before: after a certain period of time, the controller value in the PRV simply disappears. 
 
People have suggested that it doesn't matter, because once set a controller value (sent to a VST instrument) will not change.
 
Here is how it matters:   should you begin playback AFTER that disappear time, the controller will not in fact be set at all.  This applies to patch changes, too, since they are controllers.   Therefore:  you work on a portion of a project, thinking all is well, and then Render the entire project:    because of the look-back failure, you now get a truer representation of the midi data INCLUDING all that data that had faded out without your knowledge.   So -- yes, the actual data in a full render is accurate; the problem is, what you may have been working with in a portion of a project is NOT accurate.