Why does altering a table and adding a foreign key create an extra constaint?
The only way I can think of that a DDL statement like yours may result in an "extra" constraint is when there's a DDL trigger that creates that constraint in response to your DDL statement(s). If that is not the case with your database, the said extra constraint is definitely the result of a totally different statement unconnected with the one that creates the foreign key.

It could have appeared at an earlier stage (e.g. when the table was being created), or it could have been added by a statement that was executed in the same script with the one in your post and that fact for some reason didn't register with you at the time.