Not getting accurate Cardio Fitness Score readings-Versa 2

[SunsetRunner]

How is it possible that yesterday, my CFS was about 36, but now, after my best run in about 2 years (have finally made it all the way back from an injury), in one day, it dipped to a 32? Talk about a buzz kill. Or is there reason to believe these measurements are totally inaccurate at times? Any, honest info would help. Thanks. 

Moderator edit: updated subject for clarity

[EdsonFitbit]

Thank you for visiting the Fitbit Community, @SunsetRunner.

Thanks for the information provided. I'm glad to hear that you're recovered from your injury.

There are some factors that can affect the Cardio Fitness Score which is determined by your resting heart rate, age, gender, weight, and other personal information. For best results, please make sure your weight is correct in your Fitbit profile. Also, wear your Fitbit Versa 2 to sleep for a better resting heart rate estimate. Your cardio fitness score will be shown as a range unless you use GPS for runs. Increased exercise may help you improve your score by up to 20 percent over a period of two to three months

Whether you've followed our recommendations to track and improve your Cardio Fitness Score in this site without luck, please restart your watch by following these steps. In addition, please make sure to have the latest version of the Fitbit app (3.41.1 iOS or 3.41.2 Android). For instructions on how to check your current version and update, please visit this article and open the menu "How do I update the Fitbit app?".

If you're running with GPS, please try the following tips for the best estimate:

• Go for long runs (at least 10 minutes long) on flat terrain.
• Complete multiple runs to help improve accuracy.
• Run at a faster pace if you’re able to do so. You don’t have to run at maximum speed but higher-intensity runs provide a more accurate estimate.

Hope this helps.