The same question could be asked of all psychotropic drugs, in that there are families of drugs which target specific brain regions and receptors, and yet have different effects.
There are a number of complexities to take into consideration such as those highlighted above, and once equated for, you can begin to investigate the connundrum you pose . However, in terms of the comparison of the effects of drugs that affect the same receptor, there is one answer that is plainly obvious to me: if you are dealing with a different drug, you are dealing with a different molecule, and differing molecules have differing properties of interaction with other molecules at the atomic level - in this case, complex molecular structures which we call proteins that make up the binding ports for nerve receptors.
Specifically, if we keep one thing constant - the receptor (sub)type - and vary the drug molecule for the same target receptor, we can see that the variables have different 'affinities' for the constant. The variables also can be seen to have changed kinetics and dynamics for the constant; in vitro pharmacological studies over the decades show us the vast differences between these properties of 'similar' drugs.
The dynamics and kinetics of molecular biology is a whole area of study in itself, and is far too deep to go into in a post, however, I highly recommend that anybody interested in the reasons why different drugs - which at a superficial glance appear to have similar physical properties - actually behold a vast spectrum of activities at the atomic level, research this area thoroughly.
