The general idea is that the Qur'an was revealed in Arabic: Any English translation, no matter how accurate, is the work of man rather than the Unfailing Word of the Almighty.

It is common to lampshade this by pointing out that, as a translation, this is in fact only an *interpretation* of the meaning. Given that the Qur'an emphasises the importance of speaking only the truth when speaking of Allah (e.g. [Al-A'raf 169](http://legacy.quran.com/7/169), [Al-Nisaa 171](http://legacy.quran.com/7/169)), this is *especially* prevalent where the wording could imply the direct words of Allah otherwise (such as when quoting the Qur'an).

In other words, and using the example provided in OP, Allah *did not* say "Whatever of good reaches you, is from Allaah, but whatever of evil befalls you, is from yourself" (or if He did, we don't know about it so can't really claim so). He *did*, however, say "ما أصابك من حسنة فمن الله وما أصابك من سيئة فمن نفسك وأرسلناك للناس رسولا وكفى بالله شهيدا".