I used to experience something like this too.
It's almost like when somebody says "don't think of an elephant", most people wouldn't be able to.
That mental strength required to not visualise the elephant is the same that is developed through quality void meditation.
Also with void meditation, I think too many people focus on trying to suppress thoughts too quickly. Silencing the mind is like stretching a muscle, you need to ease into it. In the beginning of void meditation, it's not the silence that should be focused upon, rather the gentle ushering of the mind away from thoughts repeatedly.
After say 5-10 minutes my mind becomes very supple, like a well stretched muscle. I still get thoughts but they are like gentle wripples in a lake and easy to watch rather than become involved in. If you experience this calm state after some minutes, imagine what you would experience after weeks, months or years of practice.
Good luck, I hope you find this relevant