My population estimates differ from the census'

I have been finding discrepancies in population estimates between my calculations using IPUMS data and the US Census’ comparable data downloaded from data.census.gov.

I am working on Arab American population estimates. I downloaded 2018 5-year ACS ancestry data from IPUMs for the state of Illinois. I use the fweight = perwt command in stata for population estimates. When I compare my numbers to the census, there are often discrepancies. I am looking at comparable datasets (2018 ACS 5 year).

For example, my population estimates for Arab Americans using IPUMS data were 13,156 fewer than the ancestry data downloaded from data.census.gov. In part this is methodological (I am using a broader definition than the census); however, this would mean that my estimates should be larger.

That said, the overall Illinois population derived from the IPUMS data matches the data on the US Census website.

Is the discrepancy with the Arab American count due to small sample sizes?

Any advice would be much appreciated!