The value of 9.8m/s^2 is an average estimate of the acceleration due to gravity on Earth's surface. In reality, this value varies depending on location, altitude, and other factors such as the rotation of the Earth.
As altitude increases, the distance between an object and the center of Earth decreases. This results in a weaker gravitational force, and therefore a lower acceleration due to gravity. At higher altitudes, the value may be slightly lower than 9.8m/s^2.
The acceleration due to gravity is determined by the mass and radius of a planet. Planets with larger masses and/or radii will have a stronger gravitational force, resulting in a higher acceleration due to gravity. For example, the acceleration due to gravity on Jupiter is 24.8m/s^2, while on Mars it is 3.7m/s^2.
Due to the Earth's rotation, objects at the equator are farther from the center of the Earth compared to objects at the poles. This results in a slightly weaker gravitational force at the equator, and therefore a slightly lower acceleration due to gravity.
The acceleration due to gravity is a constant value, but it can vary due to changes in the Earth's mass distribution. For example, natural disasters such as earthquakes can shift mass around and cause a temporary change in the acceleration due to gravity in that area.