Gone a while - question about Snape

[melhersheybar]

No: HPFGUIDX 154354

I am sure this question has been debated a lot, but I have been gone 
a while, so if someone could either point me to the right threads or 
summarize the conventional wisdom, that would be great.

In re-reading books 4-6, the accounts of when and why Snape came 
back to Hogwarts do not match.  Here (without page numbers, as I am 
at work), are the inconsistencies.

Book 4 - Dumbledore tells Harry and others that Snape gave up his DE 
days and came back at great personal risk to himself before VD fell.

Book 6 (I think) - Harry learns that Snape was the one who overheard 
the prophesy and passed it on to VD, leading to the death of Harry's 
parents.  Dumbledore tells Harry that it was Snape's remorse at 
causing the deaths of James and Lily that led DD to give Snape a job 
at Hogwarts.  Yet, Snape also tells Narcissa and Bellatrix that at 
the time VD fell, he was already at Hogwarts, acting as a spy, as VD 
had ordered him to be.

So I don't get it.  Is it that there was a gap in time between when 
Snape overheard the prophesy and when Wormtail turned traitor, and 
during that time Snape went and repented to DD?  But that still 
doesn't work, because DD makes it sound like it was James and Lily's 
actual deaths that caused Snape to feel remorse, so there is no way 
he could have already been at Hogwarts when VD fell.

Am I missing something?  Help please!

mhersheybar