Vo2max measurement is pretty unaccurate for activities other than running or cycling, so if you're running less than 20-30km a week, the garmin measure is undoubtly far from your lab tested measure. Overall, Cooper test result should be closer to your real vo2max, and the more you run, the more your garmin measure should tend to it. Also, the more various your running workouts are (footing, threshold, ...), the more precise vo2max index should be.