The earth's tilt angle is forever changing and goes through cycles. Admittedly there are very long cycles but cycles never the less. That being the case the area of the tropics and temperate regions are changing constantly.
The reason why the obliquity is changin is because of the torques in the earth's orbit and the effects of the planets on earth.
equatorial bulge
Precession of the earth's axis
Polar wander
Precession of the equinoxes
Milankovitch climate cycles
ice ages
The precession index
Eccentricity of the earth's orbit