Snapedom

Fearlessness is a mere accident of temperament, whereas courage is the quality (together with integrity) on which all other moral qualities depend.

Oh, yes, as we see in Lupin. So which do you think Lily has?

*

I interpret Slughorn's "Even you, Severus" as indicating that Snape was the better student

Er. Well. Actually, neither Mary nor Sluggy ever notices (Lizzie might have), but....

My own view has long been, remember how Sluggy kept harping on how Harry's performance in Potions class was JUST LIKE his mother's? So what was Harry's actual performance in Sluggy's class? Harry was an excellent brewer when he bothered to actually follow directions and had good ones to follow. He had rather poor theoretical understanding of the subject (witness his confusion about Golpalott's Law, which seemed to me and Hermione to be perfectly clearly stated). So, working from the Prince's book, Harry turned in lacklustre essays and perfect brews. And Sluggy interpreted this discrepancy as Harry having an "instinctive" grasp of the subject which he couldn't verbalize but which came out when you put him in front of a cauldron.

Just like Lily.

I hypothesize it's for essentially the same reason. Sev's running two sets of experimental modifications simultaneously here, and he routinely gives Lily the one he considers to have a better chance of success. (Just as, in canon, he regularly and without fanfare giave his students his modified formulae to follow: in HIS classes, Hermione, following the instructions on the board, routinely did well.)

Since Mary and Sluggy never catch on, I don't really expect readers to. But since the matter teases you....