I think you're confusing Received Signal Power (which is a negative value measured between -99dBm and 0dBm) and RSSI (which is a positive value between 0 & 255 and has massive variances in implementation between manufacturers). Maybe you've used some products that report RSSI in dBm but they are incorrect in their use of this term for that measurement. Due to the massive variances in implementation between manufacturers, a true RSSI value is not at all suitable for AP placement. There's probably a case for implementing received signal power, measured in dBm, but that might lead to poor AP placement as people don't realise that a device with a low signal strength may be perfectly usable in an area of low background noise. Similarly, a client with a high receive power might not be at all usable in a noisy environment. SNR is the correct metric to display in a signal quality bar graph. It's true most devices show both signal strength and signal quality bars, but signal quality is the one that actually matters so overall it probably leads to better AP placement if used exclusively.
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