Yep, "bars" are not a great way to track signal, in general, and there's no standard for them, the OEM/OS makes the call as to these thresholds.
A bar can mean anything, from say a 20-25% threshold per, to say a 40-100% range, with 10-15% variations, it's a pretty blunt way to actually track signal values, in any appreciable way.
There's a handy little app called "Network Cell Info Lite" (I have zero affiliation, and the Lite-free version works fine for most functionality like this), that will ,let you walk/drive/whatever around, and watch the precise dBm values, to get a real idea about signal strength.
There's a mode that will track it as well, but it's pretty tough on the battery-life, IME.
As such, make sure to use the actual "stop" setting on the app (little international power standard icon, upper right) when you're done, or it'll churn away in the background, again, that pesky battery issue.
I've used this for quite some years now, to precisely determine the actual values, and to-boot it can help you tune a WiFi system as well, particularly if you have multiple APs, or similar (there are better apps for this, but the basic functionality in the WiFi side works fairly well for basic tuning).