I came across this gem while looking for something else....
WT 81 3/1 pg 29:
WHY ADJUSTMENTS IN UNDERSTANDING?
Some may ask: ‘If Jehovah’s Witnesses are getting “food at the proper time,” why have their views on certain teachings changed from time to time?’ The Bible answers: “The path of the righteous ones is like the bright light that is getting lighter and lighter until the day is firmly established.” (Prov. 4:18) Even as the congregation in the first century had only partial knowledge, so it is with the “faithful and discreet slave” today. Knowledge of many spiritual matters will become complete only as God’s purposes are worked out in completion. (1 Cor. 13:9-12) The “slave” is not divinely inspired but continues to search the Scriptures and carefully scrutinize world events, as well as the situation of God’s people, so as to understand the ongoing fulfillment of Bible prophecy. Because of human limitations, at times there may be an incomplete or incorrect understanding of some matter that may require correction later.