Lucky or unlucky?

The Diet Coke bottles in the vending machine here advertise that one out of every four 1L bottles will win a free 1L bottle. At this point, I have purchased 10 bottles of Diet Coke here and none of them are winners.

Assuming that winning bottles are randomly distributed, the chances of this happening (i.e., none of them being winners) is just under 6% (0.75¹⁰ == 5.63%).

I am starting to get a bit suspicious about this contest.

Maybe such contests are void here and they've "pre-voided" the bottles for us? That would be even sneakier...