21 days is just a base, some chicks hatch early because the egg may have been under a hen longer than another egg, or one egg could have been stored longer.
Some people see a pip on day 19 and get excited "egg is hatching!" which is true, but
the zip may not happen until 12-24 hrs later.
The original pictured egg is still Not hatched, looking about the same if not a teensy bit more zipped. I however have 1 completely hatched chick & another egg pipped.
When counting hatching days I count the first set day as a 0.
Yes I do the same.
I think with my duck hatch I will adjust my incubator to match the external/separate thermometer. I had the same thing happen last hatch so now I am confident my incubator is the issue.I would trust your thermometer in the the incubator for sure, and not just what the digital one says on the incubator. I have adjusted my incubator temp to make sure the one in the incubator has the right temp.