It is possible that you may have a mild lazy left eye or the left eye may require slight optical correction. Your ophthalmologist can examine you and guide you further.
In any case, unaided vision of 6/9 is good enough to continue to function normally in most situations.
Next Steps
Have your eyes examined by your ophthalmologist who could explain the reason for this and guide you further.
Health Tips
Keep a watch on your vision and report to your eye doctor immediately if the vision continues to fall or other symptoms like pain, redness or headaches develop.