This happens because of the Language Aliases in SMW_Langauage.php

I think the assignment is used to store the the name of the property
'Code' => '_cod',

Is it correct behaviour then? Should properties never be named the same as datatypes? I mean, that seems sort of sensible, to avoid confusion...

Yes, this is correct behaviour. It has been introduced when changing the way in which record properties are declared. The old style only required types, while the new style requires properties. To provide some compatibility, properties that are named like the types have been introduced as a default (so the old record type declarations will still work without changing wikipages).

This does not mean that you should not name properties like that (though it might indeed be confusing); it just means that you cannot change the datatype of these properties to be different from their name (which would really be confusing). If you use these properties regularly, it would be useful to (redundantly) declare their type on their wiki page anyway, to clarify the situation for other editors and to ensure that the type stays the same in all future (the default properties can change their name if, e.g., the content language is changed).

Thanks for clear this up! Makes sense.

Is this in the wiki somewhere?

(I couldn't find it; so I asked: http://semantic-mediawiki.org/wiki/Help_talk:Properties_and_types#Properties_with_the_same_names_as_datatypes )