* Earth's Tilt: The Earth is tilted on its axis. This tilt causes the sun's path across the sky to change throughout the year.
* Seasons: Because of the Earth's tilt, the sun's position in the sky at midday varies with the seasons. In the summer solstice, the sun is highest in the sky at midday for the northern hemisphere, while in the winter solstice, it's lowest.
* Latitude: Your location on Earth's surface also affects the sun's position. The sun will be higher in the sky at midday for locations closer to the equator, and lower for locations further from the equator.
In short: While midday (12:00 PM) is the middle of the day, it doesn't always correspond to the highest point of the sun in the sky.
