Q: What is an overhatch and why do you advertise it?
A: Well, let's start with the idea that we literally count our chickens before they've hatched. :) The exhortation against doing that is pretty good advice... unless you're a hatchery or breeder! Then it's your business to predict how many eggs your chickens will lay and how many of those will hatch, AND how many of each sex you'll get. So because these are all estimates, we're pretty conservative in our predictions, because we don't want to disappoint our customers if possible. It's understandably disappointing if you've ordered some Silkies, for instance, and after the long wait when hatch day comes, we don't have them! Mother Nature does what she does, though. That's why we try to be conservative.
An overhatch, on the other hand, means that more eggs hatched than we predicted. So, if we were expecting to hatch 120 white female Silkies, we might allow 100 to be reserved for orders, planning to use the remaining 20-ish to fill out assortments. But when hatch day comes, if 142 end up hatching instead of 120-ish, that would mean we overhatched.
When that's the case and we hatch more than we predicted, we advertise the overhatch, because there are some chicks available to purchase for last-minute orders. In fact, this is one way you can get a "sold out" breed, by watching for overhatches. So if you've been checking and checking and a breed remains sold out, when there is an overhatch, you do have the opportunity to get some---IF you're able to act last minute.
Unfortunately, we cannot hold these extra "overhatch chicks" from one week to the next, because baby chicks must be shipped right after they've hatched in order to arrive safe and sound.