In page 4 in the 3rd paragraph it says "left on the Dursleys' doorstep 11 years before". But in the 5th paragraph, it says "he had spent ten years with the Dursleys'"
Where was he for the other year????
He means that before last year "he had spent ten years with the Dursleys'". The paragraph after that states that "exactly a year ago, Hogwarts had written to Harry'". The last year to the eleven years is the previous year.