Explanation:
The crime rate statistic is defined as the number of registered crimes divided by the number of registered residents in a country or area.
“Registered” is important here!

If you add undocumented immigrants to the calculation, the statistic is skewed:
Undocumented mmigrants aren’t registered as residents for the statistic, since they are, well, undocumented.
However, ALL crimes committed by undocumented immigrants will count towards the crime rate when they’re registered by the police.

So even if they were much less likely to commit crime than the resident population on average, the crime rate statistic would still increase. The denominator of the equation doesn’t increase by definition, because only legal residents are counted towards the statistic. But the real number of people inside the country who may commit crimes increases.

This is important to know as context when you hear people try to “prove” immigrants are more criminal than citizens, by using the crime rate statistic.