The average "12 volt" battery usually retains a charge of around 13.8 volts be it automotive or aircraft. Automotive manufacturers have always chosen to call it 12 volts and aviation has always chosen to call it 14 volts. Same goes for the "24 volt" vs 28 volt systems. Good question though. I have been an A&P for 30 years and only got that answer explaned to me about 10 hears ago.