Perhaps they're going by the general release date. It premiered at the Globe Theater in New York on December 20, 1946 but went on general release on January 7, 1947:

https://en.wikipedia.org/wiki/It%27s_a_Wonderful_Life#Critical_response

but as also stated on that page: "The film was supposed to be released in January 1947, but was moved up to December 1946 to make it eligible for the 19th Academy Awards held in March 1947"

However, a 75th anniversary Blu-ray was released last year on November 16th 2021:

https://www.blu-ray.com/movies/Its-a-Wonderful-Life-Blu-ray/299659/

So I guess both years (1946 and 1947) are 'correct' depending on your interpretation. I'd be inclined to go with the 1946 premiere date though because that was when some of the public first saw it.