Not a biologist but I don't think it has to do with how much the genes get expressed but rather that they do get expressed and in bad times (pesticide) they're really good and look like they dominate (in that you only need one copy to get the expression/benefits) but in good times (pesticide free) they don't introduce that much negative cost and so you need two copies for adverse effects.

A beetle allele is dominant white and recessive black. It is found on white sand. This beetle ends up on a black sand beach and proliferates. Birds see the white beetle plain as day and eat them. The allele frequencies of the black recessive beetle on the black beach surge as a result to the point where it is the more frequent allele observed.