Oklahoma Statutory Interest

In reviewing royalty payment check detail for last year, I find a number of line items with dates all the way back to 2005. Inquiry to royalty owners relation department for this big boy oil co requesting statutory interest be paid, they contend these line items are adjustments and are not subject to statutory interest. Can find nothing in statutes giving them this out. It is true that I did receive royalty payments in the past for the periods in question, but these adjustments on one check equate to normal 12 months royalties. I have requested letter from them reflecting their position.

Anyone got a clue on this?