Even a perfect dead drift of a dry fly does not mean that the fly will not float at different speeds and different directions depending upon the surface current(s). In fact, that IS what a dead drift is supposed to do. Float as the natural does.
The same applies to subsurface forms of insects. They get buffeted about based on currents and obstructions. Just as one intends in a dead drift - to behave as the natural does.
Behavioral Drift is the accepted phenomenon which keeps our streams somewhat evenly populated by insects. The adults tend to fly upstream to lay their eggs. If it were not for the fact that nymphs and pupae tend to drift downstream, all the lower sections of a river would no longer have hatching insects. So, it is this natural behavioral drift of the nymphs and pupae which distribute the insect downstream.
Anyway, those are my beliefs.