casting behavior of oids and relation names

Hello guys,

In some cases when I cast the oid to relation names ('xxxx'::regclass::text)  I get  schemaname.tablename and in some cases I just get tablename. I thought at the beginning, this is due name duplication of tables in different schemas but it seems not.  Also, this seems as a schema option because it happen only in certain schemas.  How I can get a consistent names of the casting ? 

Thanks in advance