Lose one, replace with two, no three.
Go to the feed store for supplies, two bantam chicks find their way into your "supplies" for the trip home. (Bantams are so small, surely it takes two of them to equal one chicken, right?).
Oh, wow, I didn't know there were DARKER eggs than Welsummer - gotta get me one of those BCMs!
Hey, that's new, the feed store never carried THAT breed before!
Sale? Buy three and get one free? You betcha! Now, which one...wait, I already have five picked out... Just one more and I can get TWO free chicks!
I'm tired of getting straight run bantams and ending up with Roos... I am going to HATCH what I want! (Riiiight.... It's still straight run! Only now there are more of 'em, both cockerels and pullets.)
Just a few examples of Chicken Math.