I know what happened, but I still do not know how the error escaped us.
When we create a system, it is systematically designed and individually tested. We test the outputs of that system. We verify where in fact the information subsequently goes, which system takes over, and so on. All that is done systematically when we prepare for and conduct the census.
For the moment, I cannot tell you why we did not detect the error when we tested all those systems. However, we take measures and use matrices to test all these processes. Once the data are produced, they are validated. At the validation stage, we saw that changes had occurred, but we did not understand that the verification should have been done before releasing and correcting the data.
This type of error is highly unlikely but not impossible.