It is not a mistake. The Protestant translations systematically substitute one word for another in order to make the scripture sound Protestant.
Especially the NIV. It changes whole snetences! However, systematic substitution of words was and is not something only the Protestants enagged in.
I said it was a mistake because the New King James Version (NKJV) corrects Eph. 20:10 with a correct translation. It is very possible that the translator was simply doing what so many others have done when faced with a difficult verse: simply translate it to fit doctrine by picking a "better" word.
By the 16th century, Bible scholars were becoming aware of the variations in biblical texts, but it didn't really become a full-blown scandal until John Mill published his Apparatus on the New Testament in 1707 with some 30,000 documented variants (he left out minor ones such as word order which is not significant in Greek) based on mere 100 Greek manuscripts! Today, we know of about 57,000 manuscripts and a minimum of 200,000 variants.
Most people who read modern-day Bible versions have no clue how these Bible versions came about; most of them don't even realize that they are reading a translation or that it might contain errors.
They trust that centuries of transmission produced flawless copies, even those made by hand, that the well learned men who worked on these versions knew exactly what they were doing and, ultimately, that God himself made sure their work was not in any way different from the original.
It really does take a child-like naïvete to find peace that way.