They can duplicate a lot of the digital stuff live, but there is a time lag..you may notice it sometimes on the big screen at a larger stadium type show, that does not optimize repeater technology.
Ashlee Simpson, like most of her generation, sings to a prerecorded "sync track" of her own vocals to create a more lush layer of sound. All live "rap" shows are done this way as well!
there is a certain degree of timing involved and the SNL show was the dead give away. SNL used to be the only place to see "all live" music on TV, now its gone too!!! sniff, sniff
long live mediocrity!!!