So I could not lay the connection immediately, but if you're on the "concentrated scenario" your observation is completely correct (according to the info from Ids), and my "impossible" incorrect.
Understood, I just wanted to note that the IB website seem to say something else. If you read it you would expect this to be applicable for IB PM, not the OCC description. If I fill in SPY in the "Stock Margin Calculator" it indeed gives approximately 9%.