A lot of the times miners sell less than what they mine, because the exchange rate is higher than required... It's only during halving period when the rate usually is less than they need.

Lately when institutions have bought Bitcoin, they have bought it favorable exchange rate as they want to buy as much Bitcoin as possible, so they can't bargain with the exchange rate... Even if they buy solely OTC, the coins are still off the potential amount that would have been sold on exchanges.

A lot of people think it's the highest sell that matters, but real traders know it's the average exchange rate that makes things either profitable or not...