Here's why:

* Earth's Magnetic Field: Earth's magnetic field is generated by the movement of molten iron in the Earth's core. This field isn't perfectly uniform.

* Magnetic Poles: The magnetic field is strongest near the magnetic poles. While the geographic South Pole and the magnetic South Pole aren't exactly aligned, Antarctica is much closer to the magnetic South Pole than Mexico is to the magnetic North Pole.

* Magnetic Anomalies: There are also localized variations in the magnetic field strength due to geological features and irregularities in the Earth's core. These variations can also contribute to stronger fields in some areas.

Therefore, the magnetic field strength at the bottom of Antarctica is generally stronger than in Mexico due to its proximity to the magnetic South Pole.