So a couple of weeks ago I checked and saw my coolant was well below the MIN line. Today I went to buy some from the dealer, drove back home and popped the hood and the coolant was at the MAX line.

I don't know how it works but my mind is telling me logically why this would happen to me is because the antifreeze goes in wherever it goes to prevent the freeze. so when i am driving around it doesnt need the antifreeze wherever it is it prevents from freezing, thus showing full when I drove around and popped the hood.

Is this making sense?

 

The coolant (along with most everything else) will expand when hot, showing more in the tank.
The tank is often called expansion or surge tank, it give the cooling system some reserve and also a place for the excess coolant to go when it is hot.

The terms antifreeze and coolant are synomonous in this case. Coolant keeps the engine cool, but because of its chemical composition, it has a lower freezing point than water, this way, unfortunate suckers such as myself that live up north, don't have cracked engine blocks or radiatiors due to our coolant freezing in the winter.