I'm curious why an energy company would alter recorded production volumes  2 years after the oil and gas was produced. 

Based on review of several landowner royalty statements, this seems to be common practice.  Here's an example:

Well "A" produces 2000 Barrels of Oil in June of 2013.  The energy company pays an 18% royalty to "Mr. Landowner" based on 2000 Barrels and reports 2000 Barrels of production volume to the state as required.  All is well - everyone is happy.

In July of 2014, the energy company alters the production volume reported to the state, changing it to 1500 Barrels and deducts previously paid royalties from Mr. Landowner on his next check.

Later that year, the energy company changes their production numbers again, this time indicating that the production volume was 1800 Barrels.  They credit Mr. Landowner with the difference on his next check. 

A year later, in August of 2015, over two years after the product originally came out of the ground, production volume is once again altered, showing that the volume produced was 1650 Barrels. Mr. Landowner again sees a loss of royalty revenue on his statement.

If production is measured at the wellhead, how can the production volume change after it is measured, documented, and recorded?  Can anyone explain to me why this happens?