Hi Bob,
I know it seems ridiculous to think that 0.1 volts would make a difference in this application, for the vast majority of cases 5 volts is sufficient for the board to perform correctly but as @James46717 has documented you will get a rather annoying error message if the Pi notices the lower voltage supply.
My suspicion is that this was done to encourage people to use the official power supply that has some extra current delivery headroom knowing that it would be used in schools and with beginners unfamiliar with power ratings. It’s all well and good to know the Pi’s power requirements but when the end-user will inevitably connect a USB hub and a large number of USB peripherals that also need power via the Pi’s power supply the official power supplies are designed with allowance for that.
The documentation does list an assumed amount of extra USB peripherals are connected:
Interestingly they also show that the low-voltage warning is designed to be triggered at 4.63V (+/- 5%).
