Riddle me this - Export Audio Mixdown

I record mixes for a live show to an audio track in Nuendo.
I then solo, the mixdown track and export to an mp3 file, to distribute for rehearsals.
The project will have various VST instruments, midi tracks and audio tracks.

Now, sometimes, Nuendo tells me “Active plug-ins, you will have to export in realtime”…or words to that effect.
I then have to switch off the VSTs, and the export can happen offline.

But just as often, it will go ahead and do the offline export, without having to switch off the VSTs…

Is there a logical reason why?


You only get forced to export in realtime if you have in use external plugins ins your project, routed to/from some hardware in/out.
Otherwise I never had this.