Royalty payment question

I’m looking for some advice on unit price paid to the royalty owner and why is it so different from one company. I get royalty payments from about 6 different companies for Haynesville Shale production in Harrison and Panola Counties. One particular company every month pays me a significantly lower unit price for a portion of the gas sold than the other 5 companies. The other 5 companies are within a few cents of each other every month on unit price paid. When I say significant I’m talking about sometimes up to $.70 below what the other companies are paying. This is on multiple gas units, probably around 20 different units. I have contacted owner relations to this company and was told it was a pre-negotiated sales contract to a specific buyer. They are also paying me the higher unit price on a portion of the gas that is more in line with the other companies. Some wells are showing multiple prices for the same month of production, one closer to spot price and one much lower. This has been going on for at least 2 years now. I have multiple leases, some older than others. Most of the leases basically say I get paid what the company gets paid for selling the gas. It just doesn’t make sense why a large gas company would continually sell gas for such a low price. I’m curious if anyone else has had this issue and have possible answers why this is happening.