Ok here is my thery Snape New that Voldermort was coming to kill the Potters. OK he hated James but i dont think he hated Lilly so much. Snape was working w the order at great risk right so mabe he got to godric hallowes right before Voldermort so to keep there snitch as i have said the most important member of the order they put it on Snape to save him knowing that lillys love would save Harry if all else failed.
It seened that The love thing was a grevis error in Voldermorts plan that the Potters may have knew about