Poopy,
12.4V is too low compared to standard 'proper' 12V switch mode power supplies. 13.80 is the minimum voltage it should be outputting under load. More if not loaded i.e. not powering anything.
sorry, that is wrong
a proper 12v supply does 12v, period
my ATX and brick psus are all doing 12,5v max!
13.8v is used in car equipment as 12v
most 12v stuff will handle 14v, though, but produces more heat
because there are always some linear voltage regulators too