Snape and Lily as friends- How could Harry not know?

[UNIX4EVR]

No: HPFGUIDX 172671

But they were NOT friends at school.  JKR makes it clear that after the 
sorting Lily goes to Gryfindor and Snape to Slytherin.  When they met 
after that it was one on one.  People might have known they knew each 
other before school but they were NOT friends in school.

Why would anyone even think twice about the fact that they knew each 
other as kids?  Snape let his love for her be his deep secret.