Comment by m-gasser

The 4 sides of the Great Pyramid are a bit concave and consisting of 8 flat parts, not 4. You can see this for example in the pictures here: https://www.scirp.org/journal/paperinformation?paperid=12343...

The linked paper argues, that this is for stability, but it might also be intentional and it might close the error to Phi as you calculated.