Because the northern hemisphere is mostly land mass and the southern hemisphere is mostly ocean. Land heats faster and cools faster than ocean, thus the seasonal effects are more pronounced in the data.
Same with CO2 patterns which gives a similar yearly 'breathing effect'
It shows an OSM-view as default, so this still fits this community. In the past, someone posted a news article about Apple Maps; I commented that that didn't fit the community guidelines because OSM wasn't even mentioned in the article (even though Apple Maps uses it!). Even then, I didn't delete the post so...