Why GA convergence curves continue as two parallel lines?

[poster]

I'm working on a optimization problem and using GA algorithm (in MATLAB, ga function).

As you know MATLAB plots GA result with two curves, one for the best values and other to show the mean values and when this two curves touch each others it means algorithm has been converged.

On my cause these two curves don't get along and continue as two parallel lines until the end of max Generations number and finally a premature convergence,What makes it happen?

I have tested with other parameters value but same result. On pre-processing I used Missing value,Normalized and smoothing and tested data set belongs to UCI.

GA Parameters:

MutationFcn      : @mutationadaptfeasible: 0.03

CrossoverFcn     : @crossoverheuristic : 1.2

maxGenerations   : 200;

populationSize   : 180;

Chromosome length: 33

Test Instances   : 71

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