Medical Insurance. EZ? I think not!

I recently had a surgical procedure performed. The billing is in. Due to my advanced age, I'm covered by Medicare (primary) and thanks to my career in the military, Tricare (secondary.)

The total billings were:
$179.093.39

Billings approved by Medicare were:
$178,825.61

Medicare paid:
$18,103.84

Tricare paid:
$19,735.62

The EOB I received from Tricare says that I'm responsible for:
$173.56

So here are my not-EZ questions:
1.) If Medicare "approved" $178,825.61, why did they pay only $18,103.84?
2.) What is the status of the remaining $140,812.59?

I don't really want to ask anyone that second question!