Arthur Leigh Allen was the only person ever officially named as a suspect in the Zodiac murders. Investigators collected his DNA and fingerprints. Lab results showed his DNA did not match the profile extracted from stamps on the Zodiac's letters, and his fingerprints did not match those found in Paul Stine's taxicab. Despite this, a prosecutor reviewed the evidence and said the DNA exclusion was not enough to rule Allen out. What specific forensic argument did investigators use to explain why the physical evidence might be wrong rather than Allen's innocence?