Multiprocessor evolutionary algorithm consistently finds worse solution than single threaded

Not sure if this is a feature or a bug, but would love some input either way. Whenever I replace map to run DEAP parallellized as in code at the bottom I consistently get worse fitness development than with a single threaded version of the same code (just commenting out the dask_map line). These results have been replicated with both threading using multiprocessing.dummy and Dask over 5 runs each on 2 optimization problems.

What causes this consistent difference of 400 fitness between a single threaded and parallellized version of the algorithm?

Notice below fitness is minimized

Example fitness single threaded:

Example fitness Dask multiprocessing with 2 CPUs:

Setup of evolution:

ea.creator.create("FitnessMin", ea.base.Fitness, weights=(-1.0,))
        ea.creator.create("Individual", list, fitness=ea.creator.FitnessMin)
self.toolbox = ea.base.Toolbox()

        self.toolbox.register("attribute", random.randint, 0, self.MAX_NUMBER_OF_CLUSTERS - 1)
        self.toolbox.register("map", self.dask_map)
        self.toolbox.register("individual", tools.initRepeat, creator.Individual,
                              self.toolbox.attribute, n=self.genome_len)
        self.toolbox.register("population", tools.initRepeat, list,
                              self.toolbox.individual)

        self.toolbox.register("evaluate", self.eval_dsm_min)
        self.toolbox.register("mate", tools.cxTwoPoint)
        self.toolbox.register("mutate", tools.mutUniformInt, indpb=survey.param_indpb, low=0,
                              up=self.MAX_NUMBER_OF_CLUSTERS - 1)
        # Use elitism + normal ranking
        self.toolbox.register("select", tools.selTournament, tournsize=survey.param_tournsize)

Dask map function used:

    def dask_map(self, func, iterable):
        bag = db.from_sequence(iterable).map(func)
        return bag.compute()

DEAP / deap

Multiprocessor evolutionary algorithm consistently finds worse solution than single threaded #439