Here's why:

* Oxygen is soluble in water: Oxygen from the air dissolves into water, although not in very large amounts.

* Boiling doesn't remove all oxygen: While boiling water drives off some of the dissolved oxygen, it doesn't remove all of it. Some oxygen remains dissolved even at boiling point.

* The amount of dissolved oxygen decreases: Boiling water reduces the amount of dissolved oxygen compared to room temperature water, but it doesn't completely eliminate it.

It's important to note that the amount of dissolved oxygen in water can vary depending on factors like:

* Temperature: Colder water can hold more dissolved oxygen than warmer water.

* Pressure: Higher pressure allows more oxygen to dissolve.

* Other dissolved substances: The presence of other substances in the water can affect the amount of dissolved oxygen.

So, while boiling water does reduce the amount of dissolved oxygen, it doesn't completely remove it.