A wonderful explanation of the "Kri U'Chtiv" phenomenon is given by Rav Mordechai Breuer (See the article "Emunah U'mada B'Nusach HaMikrah" in the book Shitot HaBechinot Shel HaRav Mordechai Breuer). In a nutshell, there are two ways the Torah was passed down: As a written document and as a oral teaching. (See Devarim 31:19 "Now write this song for you and teach it to Bnei Yisroel. Put it in their mouths...". Also compare Devarim 31:24-27 when Moshe hands over the written Sefer Torah to the Levi'im and Devarim 31:28-30 where he gathers the Elders to "speak these words in their ears"). Over time mistakes can happen while writing a Sefer Torah. Halacha determines that we decide the correct version by the majority of the Sifrei Torah. The Oral teaching of the Torah continued in the way they received it from their fathers, while the written tradition was established through the Halachic process. Today we continue the written tradition when writing Sifrei Torah and we continue the oral teaching when reading the Torah.
I also heard in the name of Rov Solevechik that some of the "Kri U'Chtiv" in Megilat Rut differ between what Naomi says and what she wishes. Does someone know a source for this?
