It also may depend on peering arrangements Hetzner has. If EU ISP more inclined to peer with Hetzner than US one bandwidth in EU will be cheaper for them.
- in EU there are large peering exchanges to swap traffic
- in USA no peering exchanges exists and you need to pay for your traffic most of the time. Few big operators in US and they enforce this.
Looks like some deal wasn't renewed and they lost a big chunk of cheap pipe or/and some of their upstream providers decided to do something with routing.