If the plan was known, why was none of it anticipated?

"the supposed 'leak' occurred in 1904, at least a year before Schlieffen finished the famous version of his plan. Samuel R Williamson writes in The Politics Of Grand Strategy that :

The story behind French acquisition of these German documents in the winter of 1903-1904 remains confused and uncertain. In 1932 Maurice Paléologue, who had been one of Delcassé's assistants in 1904, asserted that a disillusioned German staff officer had betrayed the outlines of the Schlieffen Plan to French agents. Subsequent studies have cast grave doubts about Paléologue's accuracy both on the alleged betrayal and in his summary of the new information. Certainly there was no question, as the French diplomat implied, of the Schlieffen Plan having been revealed, since the Plan did not go into effect until late 1905."
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