Why are subtropical desert (bwh) climates generally hotter in summer than tropical humid (a) climates?

There are multiple reasons as to why the subtropical deserts are hotter than the tropical humid climates, but specifically for the summer period, it is because of the Sun is directly over the subtropical deserts and not over the Equator, or rather, the sunlight falls at 90 degrees angle and angles that are very close to 90 degrees in the summer over the subtropical deserts.

Argon, carbon-14, and carbon-12