What is the difference between voluntary false confession and coerced false confession?
I would say the motivations are far different. For any investigation, there may be a tendency for the police to be contacted by people who desire for some reason or another notoriety or infamy, perhaps this is some form of attention seeking.
In a coerced confession, the suspect usually has been interviewd poorly, may be with some denial of sme basic rights such as sleep. Along with psychological pressure this creates the perfect storm and individuals especially vulnerable ones are more likely to confess to put an end to the interviews.
Therefore the motivations are entirely different. I wonder how often other forces such as obedience also play a part, considering Milgram's experiements.