This might help....
Serama mature at 16-18 weeks.
Incubation period for Serama eggs is 19-20 days.
The Serama carry a diluted lethal gene (Japanese Bantam Ancestry), which means 1 to 2 percent of embryos will develop fully but fail to hatch or the chick will die within 24 hours of hatching.
Serama are not color bred, nor do they breed true to any one color. It is not uncommon to hatch as many different colored chicks as there are eggs that hatch.
Serama do not breed true to size. Out of a clutch of 10 chicks, one can expect 1 or 2 to be very small, 2 or 3 to be rather large and the remainder to be within the normal size range for serama.
........which would explain my latest bator trial...17 eggs only 5 hatched.