Most countries don't extend citizenship to illegals. No driver license, no housing, no benefits. They are irrelevant to the statistics- it would be like counting squirrels.
Undocumented residents are pretty different from squirrels. They participate in the economy pretty similarly to documented residents. Consumption, transportation, jobs, housing, etc, and all the related taxation and resource utilization.
>They are irrelevant to the statistics
"Wow, why are the roads wearing out twice as fast as we expect?!"