Pokémon Sword is outselling Pokémon Shield in Japan

According to Dengeki Online, Pokémon Sword with 1.19 million copies is outselling Pokémon Shield with 795,000 copies sold in Japan.

The figures for both titles released on November 15 show that for every three copies of Pokémon Sword sold, two copies of Pokémon Shield were sold as of December 26th.

What is the cause of this 3:2 sales ratio?

[Source]: Nintendo Soup: Pokémon Sword to Pokémon Shield Sales Ratio is 3:2 in Japan.

[Via]: Dengeki Online: 【週間ソフト販売ランキング TOP50】『ポケモン ソード』が16.1万本でトップ（12月16日～22日）.