It's based on how similar someone is to someone else and some people have closer matches than others. Fictitious Example where your listing top 5:

  Adam and Bob are a 80% match.
  Bob and Ted are a 90% match.
  Adam has 1 match better than 75%, but Bob has 5 matches better than 85%.  

Thus Adam links to Bob as his #1, but Bob does not link to Adam in his top 5.