A pip on even day 18 is not early, a pip is not a full hatch. (Most of the time chick sits in egg another day before it hatches.) Day 21 is just a day to expect when the egg may hatch. Depends on egg size, heat, and how developed the egg was before set, and what days were counted, I always get...
the 3 that hatched plus the big white chick is 2 weeks older from another group. He is caring for the little ones. The 3 small ones are silky mixes, they were surprise which is great, since I didn't have any silkies yet, but wanted some.