The missing billions are the Bhabhanga cittas. If you remember the citta vithi process, 1 pancadvara citta vithi is followed by 3 manodvara citta vithis and wne the third manodvara citta vithi ends….Bhavanga citta starts and these billions are Bhabhanga cittas. In fact even in busiest of time when all senses are used, our mind is in bhabhanga most of the time (more than 99% of time). Just deduct 31800 (taken from your calculation) cittas from few billion cittas a second and you get the number of Bhabanga cittas per second, which is much more than 99%.
I hope I could answer your question.
With Metta