Because the relative date function that powers that label rounds to the nearest year (it also treats 360 days as a year):

    timeAgoFromMs(ms) {
        const sec = Math.round(ms / 1000);
        const min = Math.round(sec / 60);
        const hr = Math.round(min / 60);
        const day = Math.round(hr / 24);
        const month = Math.round(day / 30);
        const year = Math.round(month / 12);

From https://github.githubassets.com/assets/javascript/node_modul...

Wow! They added a markdown formatted code of conduct 40 years ago!!

They even reference "Windows Store app package directories and files"[0]. Very futurist thinking!

[0]: https://github.com/microsoft/GW-BASIC/blob/edf82c2ebf6bfe099...