I have the same issue, and I know exactly from whence it came.
In the mid 90's I made several trips to South Africa, either JFK-JBG or MIA-CPT. This was a codeshare on SAA. At some point SAA mis-coded the flight so my milage was credited wrong.
After several letters and phone calls this was the final result.
My current ending balance is 85922
My total available award milage is 93980
The difference of 8058 is exactly the milage of one of these routes.
I got tired of fighting for the correction several years ago.