Zombies BO question 10 POINTS?

why is it that the mystery box seems to giv me the same weapons, i always get the same 4-5 crappy guns for like 20 box's until i get a gun that i can use, for example, i will always get a spas12, a python, a spectre, a cz75 then an FN FAL, i always get these guns, then after like 15-20 uses of the box i seem to get LMGs and wonder weapons, is this just consistent randomness and bad luck to get the same guns, or is it in the coding to get the same guns over and over??