how to deal with this memory leak in lua?

My machine is mac mini(2011) osx10.7.4

first of all. I download the lua-5.2.2 from lua.org, unzip it, and

$ make macosx
$ make install

then i run it

$ lua
Lua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio
> 

The memory usage now is 816KB (from Activity Monitor)

And then I type code below for malloc...

a = {}
for i = 1, 999999 do
    a[i] = {}
end

The memory usage growth to 79.0MB

And then I type code below for dealloc...

a = nil
collectgarbage("collect")

The memory usage down to 25.0MB

Ok. Here is my question.

Why there are 25MB left?

How to release or recycle them?