Question about Harry Potter and the Deathly Hallows?

Remember that Snape is still allie with Dumbledore and they both planned everything out, with consequences on their arms of course. Voldemort still believes that Snape is in his side, and for that advantage Snape himself allows that plan to take affect he imperious Mundugus and told him to go tell the order of phoenix to change the date in which he and Dumbledore planned, and that's when Snape told Voldemort.