Why do fridge temperature standards between US and other countries differ?
As a starting point, I found this article, which says:


  The suggested temperature specification for refrigeration of foods has been revisited from time to time as knowledge and technology have advanced. Initially 7°C (45°F) was considered the optimal temperature; however, technological improvements have made it economical to have domestic refrigeration units working at a temperature of 4-5°C (40-41°F). For perishable products ≤4.4°C (40°F) is considered a desirable refrigeration temperature. ... Even these measures cannot control all pathogenic bacterial growth. For example L. monocytogenes, Yersinia enterocolitica, Aeromonas hydrophila, B. cereus and C. botulinum will multiply at recommended “good” refrigeration temperatures (5°C [41°F]). There are other bacteria (Salmonella spp., E. coli and S. aureus), that, although unable to grow at temperatures below 5°C (41°F), will take advantage of temperature abuse and grow. ...


If you're interested in additional detail about all that, the article does have a bibliography, but most of it is not accessible online. I did find this table of temperature ranges for growth for several common bacteria, though. Note that for example C. botulinum grows down to 3.3C/38°F, and L. monocytogenes (listeria) grows down to 29.3°F (-1.5°C)! So even 4°C isn't stopping everything.

In any case, there definitely is concrete benefit from the 4°C guideline: many foodborne bacteria will definitely multiply above 5°C, which would lead to shorter safe refrigeration times (or more foodborne illness if you times aren't adjusted) at the 7°C or 8°C temperatures used in Germany.

I don't know anything concrete about why Germany has chosen to tolerate this risk, however. The historical progression to lower temperatures cited there does suggest that Germany simply hasn't adjusted their standards lower as technology has improved (as mentioned in the article) but it's not clear why.

A side note: outside of food safety, the primary way fridges can have an undesired effect on food is by accidentally freezing it. (Temperatures vary by position in the fridge, and by time.) There's a balance here between reducing temperature as much as possible for safety, and keeping far enough above 0°C that variance doesn't routinely freeze your food. Given that some things grow even below 4°C, I'm guessing that if it were possible to hold temperature more constant, we'd actually see even lower temperature recommendations, in the same vein as previous decreases in temperature recommendations.