When it was printed they had to estimate the publication date. In this case, publication must have happened before the estimated date.
In general, publishing a book is a lengthy process involving the publisher, the printer and distributors. When the text is finalised, the time before it is available to buy can only be estimated. The printer may promise a delivery date, but early delivery is likely to be welcomed by the publisher. The sooner it is available for sale, the sooner it is making money, so there are incentives to beat the prediction, and this presumably, is what happened in this case.