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

[guzuguzu]

No: HPFGUIDX 172598

With reference to DH, the Prince's Tale: Does anyone else find it completely implausible 
that that Harry would not have known if Snape and Lily were "best friends" for at least five 
years of their lives? Especially since they both hated James? It could not have escaped the 
Marauders who the Griffindor girls of their year hung around with, especially if they were 
hanging around with "slimy git" Slytherns, yet Sirius and Lupin never mention this fact to 
Harry, though they had several good places to do it: 

First, in GoF, when the trio meets Sirius in his cave, they discuss whether Snape is to be 
trusted. Sirius says that while in school, Snape hung with a gang of Slythern friends who 
all turned out to be Death Eaters, and then he actually ticks off a list Snape's school 
friends.  No mention of Lily. Of course, Lily wasn't a Death Eater, but it indicates that Sirius 
knew exactly who Snape's friends were.

Second, in OotP, after Snape's Worst Memory (where we learn that his "mudblood" 
comment is what caused Lily to completely end their friendship), Harry has a fireplace talk 
with both Sirius and Lupin. They discuss, at length, the relationships between James and 
Lily, and James and Snape. No mention that Snape and Lily even knew each other. Are we 
meant to believe that James had a crush on Lily, who had been good friends with Snape 
since before they came to Hogwarts and yet James and Snape hated each other "just 
because"? It seems ridiculous that the Lily-Snape connection would not come up.

Third, in HBP, at Christmas Lupin and Harry discuss again Snape and whether or not Snape 
is trustworthy, and Lupin actually spends a page of text defending Snape. I think the fact 
that his mother was best friends with him for years would have made a good justification 
to trust him, yet Lupin doesn't mention it. 

So, I wonder what Rowling was thinking? Was it that Snape's perspective led us (and Harry) 
to believe that he and Lily were better friends than they were? Lily actually agrees that they 
are "best friends", so that doesn't seem right. Maybe they were meant to be secret friends? 
However it's really not presented that way in Snape's memories at all. I am truly inclined to 
think it was that Rowling did not decide the exact nature of the Snape-Lily friendship 
through until she was in the middle of writing DH, but I welcome any theories to explain 
this. 

guzu