For note, module attributes are a lot like normal bindings, just saving you from typing the unquote part.

So this:

defmodule ModuleAttribute do
  @thing 1
  @thing 2
  def blah(), do: @thing # returns 2
  @thing 3
end

Is the same as this:

defmodule ModuleAttribute do
  thing = 1
  thing = 2
  def blah(), do: unquote(thing) # returns 2
  thing = 3
end

And thus this with an accumulated attribute:

defmodule ModuleAttribute do
  Module.register_attribute(__MODULE__, :thing, accumulate: true)
  @thing 1
  @thing 2
  def blah(), do: @thing # returns [2, 1]
  @thing 3
end

Is the same as this:

defmodule ModuleAttribute do
  thing = []
  thing = [1 | thing]
  thing = [2 | thing] # So this is now [2, 1]
  def blah(), do: unquote(thing) # returns [2, 1]
  thing = [3 | thing]
end

This is correct. Module attributes always have the value assigned to it in lines above the call. Basically you “hardcode” the parameter of IO.inspect at the time line 5 is compiled.

20 isn’t in the list, because you only add it to the list after reading it. Each usage of a module attribute will only ever know about values that have been assigned before its usage, pretty much like regular variables.