Answering my own question, just in case anyone was wondering the same thing. One possible explanation is that the Southern Hemisphere has a lot more ocean area,

Answering my own question, just in case anyone was wondering the same thing. One possible explanation is that the Southern Hemisphere has a lot more ocean area, and oceans have higher thermal inertia, and therefore the average temperature will oscillate less than in the Northern Hemisphere. When combining the two averages that are sinusoids in opposite phase, the one with larger amplitude will dominate, so the overall effect is that the global average looks more similar to the average of the Northern Hemisphere. In other words warmer in July-August and colder in December-January.