where are the infos from 'SHOW CREATE TABLE' coming from? It's not INFORMATION_SCHEMA
Ever since long, long, ago the info has come from the .frm file.  The I_S is relatively new.  In the future (5.8 maybe) the .frm will be replaced by an InnoDB-based table.

CHECK (and a few other things) are syntactically allowed but are not handled anywhere in the code.  (Another example is a DESC index.)  It could be that the .frm contains these strings, but there is no need for such unused information to be replicated in the I_S.