It's not just because it's crowded. Everyone I know who's worked in that DC hated it. Aside from that, storms regularly knock out the grid in NoVa.
Yeah, it's got to be much larger than the other regions, so it makes sense that we see more errors. Since error_rate = machines * error_rate_per_machine.
The whole region is down, you just calculated the chance of at least one machine having an error.
No, I calculated the error rate for the region. If us-east-1 has 5 times the machines (or availability zones, or routers, or EBS backplanes, or other thing-that-can-fail) as us-west-1, we would expect to see us-east-1 have each type of error occur about 5 times as often as us-west-1.