I read the same thing over and over again. It *could* happen but isn't the norm and to me a bigger indicator of incubation issues.
I hatch my Silkie eggs along side all my others... EE, Marans, whatever... They ALL hatch on day 21.
I always go to lock down by the end of day 18. (accidentally 19 recently once) The day you set IS day 1. And they SHOULD hatch on the same exact day of the week 3 weeks later.
Making sure to move your eggs around inside the incubator is KEY to having all your eggs hatch on the same day. Even inside circulated air bators there are temp differences throughout. So rotating to a new location helps to ensure even development of all the embryos.