Adding to and taking from

One of the first things that jumped out at me about Ecclesiastes 3 was the apparent relationship between the following passages:

I perceived that whatever God does endures forever; nothing can be added to it, nor anything taken from it. God has done it, so that people fear before him. (Ecclesiastes 3:14)

and...

I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book, and if anyone takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book. (Revelations 22:18-19)

and ...

For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven. (Matthew 5:18-19)

So, in the light of Ecclesiastes 3:14, which certainly seems like an orthodox view of things, how could someone be allowed to add to or take away from the Word, which was established by God? Curious...