random guess: the servers are reached via anycast. if you DDoS from the US, you'll take down access from the US, but not from other regions.
It's not much of a DDoS if it's from a single country.
That’s pretty invalid — I often see large attacks that are almost entirely made up of bots from one country.
I've even seen 100+ Gbps from a single ASN in a single country.
If the IA itself was DoSed then wouldn't your relay stop working?