I never thought of it this way. I'm American and I can understand fluent English speaking Europeans way better than I can understand British, unless that British person is from London. 70%? That's optimistic. I don't even think I am understanding words, only the most likely context in the flow of a conversation and the most likely way to react.
Fluent Northern European English (Belgium, Netherlands, Scandinavia, North Germany) almost blends into "normal" English to me. It is kind of shocking.
It's not "normal" English, it's specifically American English - a lot of Europeans have learned from (or at least been heavily influenced by) American movies etc., so they speak an American style.
IME as an American, London British can be quite hard to understand, especially at a high word rate (which was my experience). Yes.. it.. turns.. out.. that.. speaking.. slowly.. does.. make.. it.. easier.. to.. understand.
Fluent Northern European English (Belgium, Netherlands, Scandinavia, North Germany) almost blends into "normal" English to me. It is kind of shocking.