Perhaps an analogy, just to see what happens...
What if a person lived in a kingdom, which had a king ruling over the area, and the king's laws were effectively The Quran's laws, but not identified as such, i.e. they were the king's own and not tied to a scripture. If the person in the kingdom followed these laws and was asked why do you follow these laws? What's the possiblities?
1) I follow them because I feel they are the right thing to do, they are fair etc.
2) If i dont follow them i will be punished by the law/king
What if the king is the only one who undertakes all punishments for non-compliance, but the person has never seen or met the king, thus is not 100% sure the king exists or not, yet he still follows the laws. What does that mean?
3) He thinks it is more likely than unlikely the king exists, thus errs on the side of caution and follows the laws. Is this belief in the king's existence?
4) something else?
If we look at (1), if the law is indeed just/fair/etc, then its origin likely comes from a just/fair source. If a person follows this because they feel it is right, that means they are also just/fair/etc. Why would a person such as this not acknowledge the source in this case? I'm sure some would, but not all. So what is stopping them? What is the difference between those acknowledging the source and those who do not? Maybe pondering on this will help us understand it better.
Reminds me, conceptually, of 107:4-5.
4. So woe to the ones who go/turn towards,
5. But of their bond are unaware/heedless.