A letter published March 21 in the online edition of the journal Nature indicates that there are evolutionary reasons why some animals that mimic other more aggressive or dangerous species do so rather imperfectly.
The study conducted by a team of Canadian researchers indicated that, at least in hoverfly populations, predators impose less selection if the mimic is smaller. Thus, small mimic species need to have less fidelity with the species they are mimicking than do larger ones. The team concludes that the most likely reason for this is that the mimics are less profitable prey species and are not apt to be pursued as strongly as are larger species. So, less fidelity will do.
The team was also able to show little or no correlation of mimicry to several other theories that had been posited over the years as explanations for the imperfections. Among these, they were able to show that human ratings of mimetic fidelity are positively correlated with both morphometric measures and avian rankings of the mimicry, indicating that variation in mimetic fidelity is not just an illusion based on human perception.
See here for the complete paper.