Could you elaborate on what you mean by an "idiosyncratic" reaction. What I think you are asking is: could your metabolism of benzodiazepines differ from the statistical norm, in that you are either more or less sensitive to a certain benzodiazepine than the majority of the population? If so, than yes, it's definitely possible. Everybody's metabolism is going to vary in regards to different substances, some people have very noticeable differences.
If you are saying that in your personal experience .25mg of alprazolam = ~1mg of clonazepam, than that's considerably different than the generally encountered conversion ratio of the two drugs, but it's not that surprising. Although, clonazepam and alprazolam are equipotent, their subjective potency can be difficult to estimate due to their significantly different half-lives/onset and duration of effects.
So, basically, a lower equivalent dose of alprazolam has the potential to feel as strong or stronger than clonazepam because of its comparably fast onset and peak of effects, but a 400% difference such as what you are indicating is pretty unusual. If I somehow misunderstood your question, let me know and we can go from there.