In imperfect Batesian mimicry, the mimics do not exactly resemble their models. An example of this is the fly Spilomyia longicornis, which mimics vespid wasps. However, it is not a perfect mimic. Wasps have long black antennae and this fly does not. Instead, they wave their front legs above their heads to look like the antennae on the wasps.  Many reasons have been suggested for imperfect mimicry.
Imperfect mimics may simply be evolving towards perfection.
They may gain advantage from resembling multiple models at once.
Humans may evaluate mimics differently from actual predators. 
Mimics may confuse predators by resembling both model and nonmimic at the same time (satyric mimicry). 
Kin selection may enforce poor mimicry.
The selective advantage of better mimicry may not outweigh the advantages of other strategies like thermoregulation or camouflage. 
Only certain traits may be required to deceive predators; for example, tests on the sympatry/allopatry border (where the two are in the same area, and where they are not) of the mimic Lampropeltis elapsoides and the model Micrurus fulvius showed that color proportions in these snakes were important in deceiving predators but that the order of the colored rings was not.