A writer's viewpoint
Not to put it too bluntly, these conclusions do not follow from the (reported) survey findings, because they're not comparing like with like: the British and German book markets differ in other respects than simply copyright law.
English language rights are traditionally sold in two tranches -- North American rights (US and Canada) and UK and Commonwealth (excluding Canada). On a population basis alone, the UK and Commonwealth sector has a comparable number of native speakers of English to German speakers (in Germany, Austria, and Switzerland). When you throw in a proportion of British authors who (like me) also sell to the US/Canada market, it should be unsurprising that the revenue stream is fatter. It's hard for non-English speaking authors to gain a toe-hold in the English language market because most English language editors aren't multilingual and won't read submissions in foreign languages (much less pony up the not inconsiderable cost of translating a book). Basically, network externalities are on the side of the English speakers.