The Sheldon Brown article linked above explains it pretty well. Older Shimano 10 speed groups, for example, only move the rear shift cable 2.3mm per shift. So it seems plausible to me that you don't need shifter slippage to get a phantom shift if enough other factors combine in the right direction, e.g. slight RD misadjustment, sticky cables, flexy frame, grinding pedaling style etc.