My iphone is connected to the speakers in my living room through bluetooth. I noticed that when I check the volume using frequensee app, my phone temporarily cuts off bluetooth connection and uses iphone’s speakers instead, while the frequensee app is open. So I was getting the wrong volume for a while, before I noticed this.
In case you have the same setup, you might want to check this. Play a masked track or something you can hear on your speakers, then go to frequensee app, it will start playing the track on your phone while the app is open.