The OTC market is very complicated. When people want to buy huge amount they usually buy it below market price and then resell it to smaller OTC traders, some of the sellers are mining farms whom would like to keep the price of the BTC high but still need to pay the bills so they settled on selling below market rate. Some sellers don't have a lot of time so they prefer to do 50K transaction OTC instead of 50 times 1k transaction, It's a very interesting world out there.