How is the IPM Prediction Insight % Calculated

Summary:

I'm generating a 29 week forecast using IPM, based on a 54 week calendar year, and am trying to make sense of the "Mean Absolute Percentage Error" figure the "Prediction Insight" is giving me.

Content:

• The IPM Auto Prediction is set to calculate only one intersection in an ASO cube, which it does so without error.
• It then produces a single "Prediction Insight" with a "Mean Absolute Percentage Error" of 13%.
  

• Copied at the bottom of this post is all the dummy data in that intersection, I included the actuals column being used for the IPM forecast just in case, but the "Define Slice" area is setup as shown here to compare "IPM_Model_1" and "6+6 Forecast" accordingly for the prediction insight.