But why aren’t 3 and 9 part of the set?
A sequence can skip items in a typical series. For example, what is the next number in these sequences:
21, 22, 23, 25, 26, 27, 29, 30, ?
A, B, C, E, F, G, I, J, ?
January, February, March, May, June, July, September, October, ?