serial communications and on/off voltages
AFAIK the A, B and C standards are updates, they don't specify different voltage levels. 

Why those strange, inverted levels? The RS232 electrical characteristics are VERY old, and were originally chosen to drive electromechanical terminals (teletypes). Those levels were appropriate for that technology. 

After that, why change? For the same reason, we still have a keyboard layout that was designed to prevent fast typing...