When a document gives the name of an informant, consider how likely they are to know the information they are providing. Sometimes the informant may not be all that informed.
A daughter-in-law who is the informant on a death certificate probably does not have first hand knowledge of the deceased individual's parents. And yet, she may be the only person who is available to give the desired information.
Remember that even you are not a truly primary source for your date and place of birth. Your knowledge of that event is because you were told it or you read it on a document. It is not because you were aware of the event at the time it took place. Not being a primary source does not mean you are wrong, just that your knowledge of the information is not direct.
On most death certificates for someone who died and the end of a typical lifespan the informant is not someone who was around at the beginning of the person's life. That can make a difference in how accurate that information is, especially if the informant only knew the person during the last few years (or months) of their life.