CST - Why the result of the simulation is wrong on my desktop?

I simulate a antenna. For the same model, on my desktop, the directivity of farfield result is 1.87. But on my another laptop and another desktop, the directivity is 6.77. And the fact is that the result of 6.77 is right. Do you know why the result of the first desktop is wrong? What cause it? The model is absolutely same! 

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