The value "12" as month is a misprint, shared between all of Australia (COW 900), New Zealand (COW 920), Canada (COW 20) and South Africa(COW 560). (205 is Ireland, for which the correct year and month are listed.)
This is apparent from the accompanying States2016.CSV file, as shown here filtered for the date Jan. 10, 1920 (and which contains a more comprehensive list of the COW Country/State Codes than the file named as such):
The date Jan. 10, 1920, is of course the founding date for the League of Nations.
As evidence that there is no sound reason for the use of League of Nations membership as the basis for establishing independence of the Dominions, is that India is not accorded this date, despite being a founding member of the League of Nations with Australia, Canada, New Zealand and South Africa, but rather is assigned the later date of Aug. 15, 1947.
As a Canadian I also must take offense at the omission of Newfoundland from this dataset, particularly after their sacrifice at Beaumont-Hamel:
The losses sustained by the Newfoundland Regiment at Beaumont-Hamel on July 1, 1916, were staggering. Of the some 800 Newfoundlanders who went into battle that morning, only 68 were able to answer the roll call the next day, with more than 700 killed, wounded or missing. The dead included 14 sets of brothers, including four lieutenants from the Ayre family of St. John's.
The population of Newfoundland was only 214,000 in 1914 - nearly 1/2 percent of the total population died that morning in France. Most of the regiment had enlisted from St. John's, a town of only 32,000 in 1914; it lost nearly 3% of its population that morning.
I consider the dataset, as a set of verifiable and verified conclusions, to be essentially junk:
It supplies no derivation for its key facts - an essential for every enterprise claiming to be scientific.
It has no data verification, as noted here.
It entirely omits Newfoundland and Labrador, one of the British Dominions from 1907 to 1933 which joined Canada as a tenth province in 1949. Hence it is nowhere near complete even in regards well established history a century old of even the British Empire. What else will it be missing.
The selection of Jan. 10, 1920, as the independence date for these four Dominions (particularly Canada and Australia) makes no more historical sense than would be May 14, 1900, the opening of the Paris Olympic Games at which Canada and Australia participated as "independent" countries. No-one in the British Dominions regards recognition by the League of Nations as of import, in any fashion, to the evolution of independence.
The dataset should make clear when it is making a de jure versus a de facto determination, and its criteria for both.