The answer to this question parallels that of Charles Spurgeon who, when asked to reconcile human freedom with divine predestination, said, "I never reconcile friends." He maintained that the two realities fit together. So here.
Manuscripts first. The New Testament books first circulated in hand-copied form, and hand-copying by monks went on till Gutenberg invented the printing press in the 15th century. Anyone who has copied by hand knows how easily letters, words, and even whole lines get dropped out or repeated. The New Testament manuscript tradition was not exempt from this.
Also, it is clear that some copyists facing what they thought were miscopyings made what they thought were corrections. Some of these copyists added in the margin amplifying words and sentences that the next copyist put into the text itself, thinking that was where they belonged. Because the copying was done reverently and with professional care, manuscripts vary little overall, except for the occasional slippages of this kind. Manuscript comparison reveals many passages that clearly need correcting at this level of detail.
The King James Version New Testament was translated from the "received text"—the dominant manuscript tradition at the time—and published in 1611. New manuscript discoveries have led to minor adjustments to that text, and where uncertainty remains about exact wording or authenticity, the margins of honest modern versions will tell us so. The New King James, for instance, while still following the received text, notes these things conscientiously as it goes along.
Other things being equal, manuscripts are "better" when they are nearer to the original—that is, earlier in date.
In the New Testament only one word per 1,000 is in any way ...1