"Fill the bottle with water first, and then stir it evenly."

So the bottle is half water and half milk, and the concentration of milk is 50%. There is half a bottle of water left in the water bottle.

"Fill the water bottle with the milk and water mixed in the bottle."

So the bottle is half full of liquid, the concentration of milk is 50%, and the ratio of water to milk is 1: 1.

The other half of 50% milk is poured into a water bottle, so the total amount of milk is 0.5×0.5=0.25, water is 0.75, and the ratio of water to milk is 3: 1.