The reason being, before the scrolls were discovered (1947), our earliest copies (manuscripts) of the Old Testament (‘Masoretic’ texts) were dated from AD 935.1 But the texts of the Dead Sea Scrolls were actually written much earlier than these—1000 years earlier, in fact! This means scholars were able to test the accuracy of the copying process by comparing the earliest texts found in the Dead Sea Scrolls with the texts dated much later from AD 935.
Figure 1: The time difference between the Dead Sea Scrolls and the oldest Masoretic text. Note all numbers are approximate and rounded.
And they found that the Dead Sea Scrolls were 95% identical to what was written 1000 years later. And while a 5% different might sound quite large, it actually isn’t. Let’s take Isaiah 53 as an example.
While Isaiah 53 contains 166 words, scholars discovered that out of these 166 words, only 17 letters were different between the Dead Sea Scrolls and the Masoretic texts that were written 1000 years later (AD 935).2 And of these 17 differences, ten are spelling variants (like the difference between ‘color’ and ‘colour’) and four are stylistic changes (like whether or not to include a conjunction like ‘and’).3 None of these affect the meaning at all. This now leaves us with the last three of the 17 letters to be accounted for. These three letters actually spell the word ‘light’ in Isaiah 53:11.4 The Masoretic text does not have this three-letter word, but the Dead Sea Scrolls have it. Scholars think that in all probability, the word ‘light’ (as contained in the text of the Dead Sea Scrolls) is the most likely (hence the NIV translation includes it). But in all honesty, the inclusion or exclusion of this word does not alter the fundamental meaning of the text.
All this is to say that despite 1000 years of hand-copying, the meaning of the biblical text has not suffered any substantial change. This degree of accuracy is because the Masoretic scribes (Jewish Bible scholars who lived AD 500–1000), who copied the Old Testament, would count all the paragraphs, words and letters of the book after they had copied a book, and used this counting to make sure they did not make errors. Their respect for the Old Testament was so great they simply did not want to make a single mistake.
The bottom line? The Bible was copied extremely accurately from the time the Dead Sea Scrolls were written (from the third century BC to the first century AD) to the Masoretic texts some 1000 years later. As more ancient manuscripts are discovered, we are moving closer and closer to the original documents, and this is reflected in the very slight revisions in our extremely accurate modern Bible translations.