Thanks, I dug a little deeper into that page and found more info about the books.
The series:
1 Anne of Green Gables 1908 11—16
2 Anne of Avonlea 1909 16—18
3 Anne of the Island 1915 18—22
4 Anne of Windy Poplars (US & Canada)
Anne of Windy Willows (Other) 1936 22—25
5 Anne's House of Dreams 1917 25—27
6 Anne of Ingleside 1939
It seems that Book 4 had a different name in the US and Canada to the name it was given elsewhere where it was feared the title was too similar to Wind in the Willows. According to:
https://en.wikipedia.org/wiki/Anne_of_Windy_Poplars
"The novel features a series of letters Anne sends to her intended, Gilbert Blythe, who is completing medical school. Chronologically, this book is fourth in the series, but it was the seventh book written."
More info found:
https://en.wikipedia.org/wiki/Anne_of_Ingleside
"Anne of Ingleside is a children's novel by Canadian author Lucy Maud Montgomery. It was first published in July 1939 It is the tenth of eleven books that feature the character of Anne Shirley, and Montgomery's final published novel. (Two novels that occur later in the Anne chronology were actually published years earlier. As well, the short story collection The Blythes Are Quoted, written in 1941/42, but not published until 2009, concludes the Anne chronology.)" (Irrelevant info deleted by me.)
I'm curious as to why she wrote some of the books out of order. I suppose I'm used to modern authors like Rowling who publish parts of a series according to the chronology.