Cardio Fitness getting less accurate

[_heg]

Firstly, love fitbit and my Inspire HR.

I had a Charge 2 for 12 months+ and had a cardio fitness score of 50 or 51 consistently for about a year.

I changed to an Inspire HR a few months ago so I could track swimming too.

After a couple of weeks my Cardio Fitness dropped to 49. 
Then after a couple of months it dropped again and went to a range, 44-48, rather than a specific number.

For the last 3 months now i’ve only had a cardio fitness score as a range.

I walk 15k steps per day, work out 2 or 3 times a week and do 1 or 2 30-40 minute runs every week.

My fitbit is connected to an iphone 11pro running ios 13.2 and i use Aria 2 scales every day.

I’m not sure what more I could do to get an accurate result?

Any advice appreciated!

[JohnnyRow]

From What is my cardio fitness score? 

How does Fitbit measure my cardio fitness score?

Your cardio fitness score is determined by your resting heart rate, age, sex, weight, and other personal information. For best results, make sure your weight is correct in your Fitbit profile. Also, wear your tracker or watch 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.

How do I get a more precise estimate of my score?

If your Fitbit device connects to GPS, we can provide a more precise estimate of your score using the relationship between pace and heart rate during your runs. This is because individuals with higher VO2 Max have a lower heart rate while running at the same pace compared to individuals with lower VO2 Max. If you're able to run at a comfortable pace for at least 10 minutes, track a run with GPS. We recommend running on flat terrain as much as possible since only the flat sections of your run count towards your score estimate. You may need to go on several runs that are at least 10 minutes in length to affect your score.

For instructions on how to start in the Exercise app, see How do I track my workouts with my Fitbit device?

It looks like you should get an exact number if you are doing GPS runs.  Have you changed how you are measuring your runs?

[_heg]

Thanks for the quick reply!

I haven’t changed anything when tracking a run - except my phone itself - from an iphone x to an 11.

I have location services set to Always On for fitbit. and i always have the fitbit connected to the phone before I start a run.