In harry potter 7 - the movie - why does Lucius panic so much when Voldemort asks for his wand?

i'm watching it right now and i find it quite weird. i havent read any of the books so maybe i'm missing something! the moment before he explains about how his and Harrys wands are twins - therefore he must use another wand to kill Harry, Bellatrix asks if she can be the one to kill him, but obviously V says no. so it struck me as weird when he offered to kill H with anothers wand - she didnt hand hers over immediately. . . please help! xx