We may have flawed measuring devices, but they are attempting to measure something with a real, constant value. The fact that we are not directly connected to reality doesn't mean that reality is an illusion.
If, in reality, there were zero apples in my hand, I could say that I was holding one apple, but then I would be wrong.
If, in reality, there were zero apples in my hand, I could say that I was holding one apple, but then I would be wrong.