They have the full mark so there is room for coolant from the radiator.
When the coolant gets hot, it builds pressure and pushes some of the coolant out of the radiator into the overflow. As the coolant in the engine/radiator cools it pull the coolant back (via the hose at the bottom of the overflow). If you have a leak in the cooling system it can push the fluid out and not pull it back, but that's a different issue..
So, If you have too much coolant in the overflow it will, well, overflow as it is pushed to the overflow. Coolant on the ground in front of a rear tire is not good...
Basically, just drain some fluid out of the overflow. Use a turkey baster or something similar.
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