If my memory isn't going fuzzy on this one (I read a bunch of Liu last year but didn't keep any of them), it eventually turns out that Granny's voodoopowahs aren't really voodoo -- she's just using rootwork and such to give her own magic some external structure and focus. There's a bunch of white witch characters, good and bad, who seem to have the same sort of innate powers, even though they pick different forms to ritualize it.