How can mutagens cause mutations despite the body's DNA repair enzymes?

The efficiency of the repair mechanism is not 100 percent.

Explanation:

The DNA damages caused by physical conditions (i.e., exposure to radiation, UV light, etc.) and chemical agents (i.e., mutagenic compounds) are regularly repaired by DNA repair enzymes that recognize and correct nucleotide errors before the fixation of the mutation. The Apurinic/apyrimidinic (AP) endonuclease is the enzyme responsible for repairing the damaged DNA. However, this process is not perfect, and mutations that were not recognized by this mechanism are fixed in the next round of replication.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.