In a nutshell, the consensus seems to be that the bars (properly known as the “signal quality estimate”) don’t really mean much of anything. There is no industry standard. Each manufacturer uses their own algorithm to determine the bars displayed. This varies not only between phone makers, but also between models, and between firmware versions of the same model. The number of bars displayed does not necessarily equal signal level. instead, it would be more accurate to say that the bars are “correlated” with signal level.
In addition, several other factors explained below determine the customer experience.