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Statistics
The friendship paradox
Scott Feld, Why Your Friends Have More Friends Than You Do, American Journal of Sociology, 1991.
Take any group of people and write down who is friends with whom, counting friendship as mutual. Now compute two averages. First, ask each person how many friends they have and average the answers over everyone: call this the average number of friends. Second, for each person, look at that person's friends and average their friend counts; then average that figure over everyone. Call this the average number of friends of friends.
No one is lying and nothing is being measured badly; this is pure counting on a fixed list of friendships. How do the two averages compare?