In the begining of sefer bamidbar, the males between 20 and 60 are counted and it says that there are over 600,000 of them. Theoretically each, or most, of these people should have had a first-born child and half of these children should be males. But two prakim later, the first-born males one month old and up are counted and instead of there being about 300,000, there are only about 22,000. Why are there so few?
I checked several mefarshim and none of them seemed to have an answer.