If the villagers were bad, we'd behead ten peasants.
If the villagers were good, we'd behead one peasant.
We would never behead five peasants, because that would just confuse the entire village, we'd seem indecisive, and they'd think that we weren't paying attention or that maybe we were sleeping on the job or something.
Its very hard to maintan an active day-to-day interest in the good and evil done in a village, the deeds of peasants are much more boring than they seem to think they are, I can't imagine a god being interested in each and every one of their petty little squabbles and infractions.
Luckily for us they had no understanding of fractions, or they'd want to argue about whether we were rounding up or rounding down when the tally of good and evil came to six and three quarter heads!
So usually we'd take six heads, and sometimes we'd take four, when we weren't paying attention and we weren't certain how many beheadings they deserved, which was basically all the time.
And we'd cheat on jolly holidays and just pretend they'd all been good, so only one head would need to be taken.
And sometimes we'd even take a break and sleep in on holidays and just add that head on to the next pile of heads we needed to take.
But regardless of how many heads we took, we'd do it bearing an expression of extreme confidence, as if six heads was exactly the right amount of justice to be dispensed, because that's all the peasants really cared about.
And we'd never take five.
Not even on a four-head-day after a holiday with a head carried over, the peasants wouldn't understand the complex math, its best to take six or seven heads, in that case.