I guess it's not really timing--it has to do with when the band comes in after each of Plant's acapella sections. Note how they come in at about the 12 second mark.... Then compare that to how they come in at the 1:09 mark and at 1:20--both of these his voice has barely decayed when they come in with the main riff. Whereas going back to that 12 second mark, it's almost a half second delay. My assumption is this wasn't done up "live" in the studio, so it seems certain it was intentionally done this way, versus synchronized sloppiness. Wonder why Page did it this way?