First, 48V is just symbolic. In reality, the actual voltage range might not even include 48 volts.
By nature, PoE is DC power. And historically, DC power source equals batteries.
A typical lead-acid battery has a nominal voltage of "12V", but they can vary between 10.5V (fully discharged) and 12.7V (fully charged).
Five batteries in series to power are beyond the limit because the high end may exceed 60V (5 x 12.7V = 63.5V).
Why not exceed 60V?
Because the maximum Safety Extra Low Voltage (SELV) is 60V which is to make sure that there is no risk of electrocution when users are in direct contact with the current.
By the time IEEE came to set the rules, they had decided to leave a 5% safety margin, so the maximum acceptable voltage for all PoE applications is fixed at 57 (60-60*5%) V in the 802.3af/at standard.
Therefore, we consider four batteries in series, which makes 12V × 4 = 48V (nominal). That’s where the “48V” came from.
IEEE made it clear that the maximum voltage is 57V. What about the minimum? 10.5V (fully discharged) × 4 = 42V. After adding the 5% safety margin, we got 44V as the minimum voltage in IEEE 802.3af standard. That’s why the full DC voltage range specified by IEEE 802.3af is 44 to 57 volts.