The hatching depends on lot of things, including temperature. Hens don't control temperate uniformly like the incubator.
And if the temperatures outside are high enough, the eggs layed in the beginning of clutch may hatch earlier than
others in the batch.
Twenty one days is the average incubation time for chicken eggs. They are subject to all the variables that the above posters have listed and more besides. Especially when a broody is going the hatching the actual hatch time can vary quite a lot. Be patient.