Why is there Ancestry variation between IPUMS and US census Factfinder?

For ACS Ancestry data, 1st response (2007-2011, ACS 5-year sample). I compared the summarized microdata from your site to the summarys generated by Factfinder at the US census site. I expected them to be the same, but they are not. I know groups with under 10,000 people are rolled up for privacy issues, but is that the only thing that causes these variances?

example for the whole US:

ANCESTR1D IPUMS Census summary

Somalian 105,927 102,503

Kenyan 41,960 45,089

Its also not clear exactly what Ancestry group the omited ones are rolled up to.

I think what you are seeing is due to the fact that the ACS PUMS files include only a portion of the cases used in the summary files. In fact, PUMS estimates for basic demographic characteristics such as sex differ slightly from FactFinder as well. Please see the link below for more details.