There has been much discussion around the different descriptions of the offender, leading to speculation there was more than one offender , possibly a different offender in each of the cases. In television documentary, The Hunters, journalist, Adam Shand presents a theory that Mr. Cruel did not exists and that each of the crimes was carried out by seperate individual's, partly due to varying eyewitness descriptions of the perpetrator.
Can you trust eyewitness accounts?
Margaret Berger is a key researcher in the field of eyewitness testimony. One particularly influential study she conducted involved staging a crime in a classroom setting to examine the reliability of eyewitness accounts (The role of memory in eye witness testimony). In this experiment, Berger presented a scenario where a perpetrator entered the classroom and stole and object. The students, unaware it was staged, were questioned about the event. Their responses, including details about the perpetrators physical characteristics , revealed significant inaccuracies, highlighting potential for errors in eyewitness memory.
http://www.mhhe.com/media_library/hssl/psych/psych_videotranscripts/eyewitnesses_transcript.pdf
Eyewitness false memory is a well documented phenomenon where memory of an event is significantly inaccurate. This can be caused by a number of factors, including: misinformation, suggestive questioning, confirmation bias and stress. Memory is fallible.
Further reading
Eyewitness memory : how stress and situational factors affect eyewitness recall.