Not hard to understand. The phosphorous that caused the beneficial reaction (probably electrochemical to produce the necessary hydrogen--but the article doesn't say--consider that "chemical speculation") is distributed uniformly throughout the iron matrix due to the way the iron was made. The film will only form on the surface exposed to moisture and oxygen, which is only (but everywhere) on the surface of the entire pillar from top to bottom.
And, will continue to grow at a decreasing rate as less iron is exposed to the ambient oxygen.