This confusion is shared by other Archive visitors. When you are at the main page of the RS Archive:
click on the "CoTS Verses" button in the top menu-bar. In the window that comes up, click the "About" button at the bottom of that page. The window that comes up will have many links, but click on the "A Note on the Dates in the Calendar" and you will see this text:
"The dates above the verses relate to the manuscript of the first edition, which covered the year 1912–1913. When he was questioned about the change of dates that occurs from year to year, Rudolf Steiner stressed that one must always begin with the first verse at Easter. Thus, the change in dates is not important because three successive verses of the Calendar are always kept in the same mood."