maybe numbers are not the best choice if you want randomness?
people could choose based on order (incremental/decremental, count order), or based on the physical position (center, top, bottom), or numerical position (extremity, center)
I picked 1 to "not be the centered choice". My choice wasn't random, but based on a logical attempt to counter your test or expected result.
I think If you made people choose something else than a suit, it would be more random.
a basic form (cercle, square, triangle), different colors, etc., something they can't place in the poll's position itself. I think numbers are naturally seen as a position, and naturally contains a beginning and an end.
I don't know what you were really expecting, but if you really want numbers, and if your test didn't have to contain a begining/end, I think you should choose random numbers yourself (4 27 6 ?), don't write them in incremental or decremental order. people will choose based on the number, their choice or life's experience, but not on the position anymore. you remove one possible thing which could make people pick on logic instead of real randomness.
I'm not sure you'll follow my idea. sometime I have strange thoughts