Ok, sounds strange – but let me help you understand (it’s all pretty simple).
Each and every day of my life, I have experiences from the past, and experiences in the present that change me, that mold me, that make me who I am. The Jed of tomorrow, is not the Jed of today. Because of that, when you look at it from an eternal perspective, I am not who Jed is, I am becoming who Jed is.
So what is the point you ask?
Well, when God said “I AM”, what he was saying is “I EXIST”, that is, I am the same yesterday, today, and tomorrow, he was saying that he can not change.
To explain this a little further, God, being outside the constraints of time, space and matter, is never becoming God. He is never a different God tomorrow than he was yesterday because a succession of moments and experiences (God can not have a succession of moments, because he is by the definition of God – outside of time which is required for a succession of moments, and change).
So when God said “I AM” – it was a declaration of what God is – what God has to be, it is so far from human comprehension, and yet, it gives us so much understanding about his nature. He is the creator that is not created, the mover that is not moved, He is One.
Praise be to God!