When you put a piece of bread in a conventional oven to toast it, the top side tends to toast first due to a combination of factors:
Direct heat exposure: In a conventional oven, the heating elements are usually located at the top and/or bottom of the oven cavity. When you place the bread directly on the oven rack or on a baking sheet, the top side of the bread is closer to the top heating element, which results in more direct heat exposure.
Radiant heat: The heating elements emit radiant heat, which travels in a straight line. Since the top side of the bread is more exposed and closer to the heating element, it receives more direct radiant heat than the bottom side.
Air circulation: In most conventional ovens, the hot air circulates unevenly. The rising hot air tends to accumulate more at the top of the oven, leading to higher temperatures near the top heating element. This uneven air circulation contributes to the faster toasting of the top side.
Moisture retention: Bread contains some moisture, and the top side is more exposed to the oven's heat and airflow. The heat helps evaporate the moisture on the top surface of the bread, which aids in the toasting process.
To ensure even toasting, you can try flipping the bread halfway through the toasting process. This will allow both sides to receive relatively equal exposure to the oven's heat and result in more evenly toasted bread. Additionally, using a baking sheet or rack that allows air circulation under the bread can also help achieve more consistent toasting on both sides.