Fun tech fact:
Those “bars” don’t mean service speed. They represent how close you are to the tower distance wise. Quality of service, speed, network traffic, are all not accounted for.
Alright. My WiFi says it’s at 4 bars but it’s acting like it’s at 1 
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The "distance" is measured by the strength, which generally works as an interpretation until strength fluctuates. Though it could be considered accurate enough to say the bars measure distance, it is easier, and much more clear and concise to say strength.