
Some call me Tim |

Pathfinder changed the wording of natural armor when compared with the 3.5 SRD.
SRD: "Natural armor bonus changes to a number based on the skeleton’s size: "
PRD: "Natural armor bonus changes as follows:"
Then they both list the same chart.
I always assumed that you replaced the creature's existing armor bonus with the bonus shown in the chart.
However, I'm preparing to run a Pathfinder Society Scenario that clearly shows they added the the bonus from the chart to the creature's existing natural armor.
Did the editors miss this or have I been doing it wrong.