I'll attempt to answer this question. Literally, the author is claiming that sometimes, translators do one thing, but claim they are doing another. Sometimes, they don't even know what they themselves are doing.
I think the author is saying that translators often cannot explain the process they are using. Even if they attempt to explain it, they will explain it incorrectly. Sometimes they aren't even aware of the process they're using.
Without more context I can't be sure, but I get the impression the author is saying that translation is a process that many people do "by feel"; they read in one language and write in the other based on what feels right. Perhaps the author believes this is a sign of the incompetence of many translators: they do not understand their own profession. On the other hand, perhaps the author believes this is a sign of the inherent difficulties of translation: translation is an art and any attempt to describe or explain it will inevitably fall flat.