Herd question

Why when I awake the morning after a herd event ends, are there 10 walkers at my wall rather than 30? What happened to the other 20? Did they fall into my walker pit overnight? Get bored and wander away? Quit the game since it was ruined by tools? Get distracted by roaming capybaras?
ShadowWalkerSuperheroGovernatorDiElElNovellWalkerslayer82SCBMALady JJenngStalker20