I always was under the impression that we can use column names as %%Column name%% inside an AMP script, if the column is part of the data extension(containing subscriber key) we have chosen as target audience choose has that column.
However to my surprise recently, there was an email send using a DE which only had subscriber key in it and no other columns. Yet AMP rendered %%Greeting%% in the emails.
So I am assuming that attributes associated to subscriber key in the All subscribers list are also always available.
This may be a stupid question, but can someone please explain to me how this is happening?