How does an accelerometer interpret g as ( +9.8 or -9.8 )
Think of the accelerometer as a spring and the displacement as the acceleration value (this is actually how it is done, kind of).

When you accelerate there is a force on the spring and from the displacement you can measure the magnitude of the acceleration. You need three springs, one in each direction to have a 3D accelerometer. Now we just agree that when the acceleration is to +X direction the value is positive and the same for Y and Z. We also agree that +Z direction is towards the sky.

Now lets focus on the Z axis spring. What happens to it when it is on free fall? Only the gravitational force applies, but it has the same affect on each atom of the spring, so the spring is not displaced. What happens when it is in rest? The gravitational force and the counter force, earth pushing upwards, cause a displacement. The displacement is to the same direction as when you accelerate towards the sky, which we agreed to be positive. Thus you get a positive 1g when standing still. In other words, the accelerometer is ignorant to the gravitational force. In rest it can only measure the counter force, which is upwards.