It is possible that Pharaoh was uncertain about the matter of knowledge and choice, which the Rambam (Maimonides) elaborates on in detail (Chapter 5 of the Laws of Repentance, Halacha 5), and this is his language: "You might say, does not the Holy One, blessed be He, know all that will be before it happens? If He knew that this person would be righteous, it is impossible for him not to be righteous. And if you say that He knew he would be righteous, yet it is possible for him to be wicked, then He did not know it definitively. Know that the answer to this question is longer than the measure of the earth and wider than the sea, and it involves several great principles and lofty mountains. But you must know and understand this matter that I am saying. We have already explained in the second chapter of the Laws of the Foundations of the Torah that the Holy One, blessed be He, does not know with knowledge that is external to Him, as humans do, who are separate from their knowledge. Instead, He, His Name, and His knowledge are one, and human intellect cannot grasp this matter completely. Just as a human cannot grasp and find the true essence of the Creator, as it says (Exodus 33:20), 'For no man shall see Me and live,' so too, a human cannot grasp and find the knowledge of the Creator. This is what the prophet said (Isaiah 55:8), 'For My thoughts are not your thoughts, neither are your ways My ways.' Since this is so, we have no capacity to know how the Holy One, blessed be He, knows all creatures and actions, but we know without doubt that human actions are in the hands of humans, and the Holy One, blessed be He, does not compel or decree upon a person to act in a particular way. This is not known merely from religious tradition but from clear proofs from wisdom. Therefore, it is stated in prophecy that humans are judged for their actions according to their deeds, whether good or bad, and this is the principle upon which all words of prophecy depend."

And this is why Pharaoh said, 'I do not know Hashem,' because he did not have faith in divine providence, and therefore he could not accept this matter in his understanding.