One reason why there are manuscript differences is because of simple scribble error. Sometimes a scribe of the Bible would make a simple mistake in copying out a manuscript.
“Unintentional changes of various kinds all arise from the imperfection of some human faculty. These constitute by far the vast majority of all transcriptional errors. 1. Errors of the eye • Wrong division of words that resulted in the formation of new words—early manuscripts were not punctuated, and letters were not separated into words by spaces. • Omission of letters, words, and even whole lines occurred when the astigmatic eye mistook one group of letters or words for another, sometimes located on a different line. • Repetition results in an error opposite the error of omission. Hence, when the eye picked up the same letter or word twice and repeated it, it is called dittography. • Transposition is the reversal of the position of two letters or words. This is technically known as metathesis. In 2 Chronicles 3: 4, the transposition of a letter would make the measurements of the porch of Solomon’s Temple out of proportion—for example, 120 cubits instead of 20 cubits as in the LXX (Septuagint). • Other confusion of spelling abbreviations or scribal insertions account for the remainder of scribal errors. This is especially true about Hebrew letters, which were also used for numbers and could be easily confused. These errors of the eye may account for many of the numerical discrepancies in the Old Testament (cf. 2 Kings 8: 26; 2 Chronicles 22: 2). 2. Errors of the ear occurred only when manuscripts were copied while listening to someone read them. This may explain why some manuscripts (fifth century onward) read kamelos (a rope) instead of kamēlos (a camel) in Matthew 19: 24. In 1 Corinthians 13: 3, kauthēsomai (he burns) was confused with kauchēsomai (he boasts). 3. Errors of memory. These are not so numerous, but occasionally a scribe might forget the precise word in a passage and substitute a synonym. 4. Errors of judgment. The most common error of this kind is caused by dim lighting or poor eyesight. Sometimes marginal notes were incorporated into the text under the misapprehension that they were part of the text….5. Errors of writing. If a scribe, due to imperfect style or accident, wrote indistinctly or imprecisely, he would set the stage for future error of sight or udgment. Rapid copying was no doubt responsible for many errors in writing. This is viewed especially in the parallel accounts of the Kings-Chronicles corpus.” (Ed Hindson & Ergun Caner (General Editors), The Popular Encyclopedia Of Apologetics: Surveying The Evidence For The Truth Of Christianity, 98-99 (Kindle Edition); Eugene, Oregon; Harvest House Publishers)
In other words, most of the “changes” in the manuscripts were due to simple scribal errors.
Does this mean that we cannot trust the text of Scripture? Not at all!
“The overwhelming majority of these four hundred thousand supposed variations stem from differences in spelling, word order, or the relationships between nouns and definite articles. In other words, a copyist simply switched a couple of letters, misheard a word, or skipped a line of text. Such variants are readily recognizable and, in most cases, utterly unnoticeable in translations!…In the end, more than 99 percent of the four hundred thousand or so differences fall into this category of variants that can’t even be seen in translations!” (Timothy Paul Jones, Conspiracies And The Cross, 1531-1540 (Kindle Edition); Lake Mary, Florida; FrontLine)