My Racing Game Steers Itself, Is My Accelerometer Out Of Calibration?

You have read your own test result correctly, which makes this easy. A reading of -0.09 on roll with the controller genuinely flat means the stored calibration is telling the controller that flat is nine hundredths of a g off level. Converted to an angle, that is in the region of five degrees, which is exactly the tilt you have to hold to drive straight. The numbers agree, and that agreement is what tells you this is calibration rather than a broken sensor.

An accelerometer, like a gyroscope, has an offset that is measured once and stored. It can drift over time, and it can also be stored wrongly in the first place if the calibration was performed while the controller was not actually level. That is the most common cause by a wide margin. People calibrate on a desk that slopes slightly, on a sofa, or holding the controller in their hands, and the error gets baked in permanently until the next calibration.

So the fix is to recalibrate properly, and properly means on a surface you have actually checked is level, with the controller face up, completely still, and left alone for the whole process. A phone's spirit level app is accurate enough to check a desk, and it is worth doing because this is the step everyone skips.

Two details worth knowing. Calibrate at the temperature you play at rather than straight from a cold room, because the offset shifts slightly with temperature. And if the reading comes back to near zero after calibration but drifts back to -0.09 within a few days, that points at a sensor that is no longer holding its calibration, which is a hardware matter. If it holds for weeks, you simply had a bad stored value and the problem is solved.