As you've titled your post with "Bluetooth" - I'm guessing that you're using a Bluetooth connected device when noticing this.
Most Bluetooth audio devices (and many others), have built-in frequency limiters called dBFS Limiters. This prevents the audio source from "clipping" at higher volumes. As every phone has a different dB level of output, sometimes on certain devices, their upper-limits will trigger this protection to prevent that audio clipping, so you may notice a slight reduction in volume when maxed out on the volume bar.
Hope this clears things up
