Why are tables constructed randomly?

[Luther <lutheroto@...>]

Consider the following program (in Lua 5.2.2):

-------------------------------------------------------------------------
local data = {
  Troll = 25,
  Doppelganger = 25,
  ['Dragon man'] = 25,
  Draven = 25,
  Dwarf = 50,
  Elf = 44,
  ['Death elf'] = 11,
  ['Seelie fey'] = 30,
  ['Unseelie fey'] = 30,
  Orc = 30,
  Gnome = 30,
  Goblin = 30,
  Human = 55,
  ['Lizard man'] = 20,
  Ogre = 20,
  Gargoyle = 15,
  Seraphim = 15,
  Daemourn = 15,
  halfBreed = 5
}

-- Run the pairs loop twice to show that the order of keys is identical.
for _ = 1, 2 do
  print()
  for race in pairs(data) do
    print(race)
  end
end
-------------------------------------------------------------------------

The loop shows two identical sequences of keys. However, in different
runs of the program, the order of keys changes, as if the order is
completely random. I know Lua doesn't give any guarantees about the
order of keys, but I can't think of any reason why the hash table would
be constructed differently on each run. Can anyone tell me what's going
on here?

(I know C, so you can point me to the relevant source code if you think
it will help me understand.)

Luther