The rate at which one is consuming alcohol is obviously a factor that impacts blood alcohol concentration.  As discussed above, the rate at which a given person metabolizes and eliminates alcohol is reasonably certain and fairly constant, and importantly, is much slower than the rate of absorption.  Simply stated, people absorb alcohol and become drunk much faster than they can metabolize and eliminate alcohol and become sober.  We all know this from experience: a person can go from completely sober to completely drunk in an hour, but if that same person then stops drinking, he or she will not be sober an hour later.  Since we absorb alcohol faster than we eliminate it, if we drink it (the rate of consumption) faster than we can get rid of it (the rate of elimination) alcohol accumulates and blood alcohol concentration increases.