Why is there a big difference between singing with a microphone and singing a cappella by yourself?

What people usually say is not actually heard by themselves. Most of it is transmitted to their eardrums through the bones of the body.

So the voice heard by oneself and others are different. Same.

This high school physics book has an introduction in the student source section.

A cappella singing is transmitted through bones, and the microphone sound is heard by real ears.