> is there any technical reason why us accounts might be identified as foreign

Speculation: they're resolving historical IP addresses against a current IP geolocation database. An IP which belonged to a US company in 2010 may have since been sold to a Nigerian ISP, but that doesn't mean that the user behind that IP in 2010 was actually in Nigeria.

I was going to speculate the same thing. Seems pretty likely... a lot of systems record information about when an account was created, and IP location correlations are period sensitive (at least some of them are... my college is most likely still using its /16 and hasn't moved outside the city where it was founded)