Deceleration is a negative acceleration, not a reduction in acceleration.
Deceleration is therefore a reduction in velocity (which is the intuitive bit).
When I get confused about this, I think about driving a car. When the foot is on the accelerator, you accelerate. When the foot is on the brake, you decelerate. So that graph basically shows someone who starts off with their foot hard on the accelerator, then slowly lifting off, then slowing pushing harder on the brake. Basically, it's a drag racer. I find using real examples like this are helpful to figuring out what kind of motion a particular graph is representing.