In the city I grew up in (in Bavaria) every store has to close by 8pm and no store can open on Sundays. The exception are bakeries and gas stations. (I also think that stores inside of train stations are exempted. And just so there is no misunderstanding, restaurants, cafes and pubs are of course also exempt from this.)
Where I grew up there is not a single open grocery store anywhere after 8pm or on Sundays for hundreds of kilometers. That makes a difference to places where stores can legally remain open longer.
I’m currently living in Thuringia where stores can be open until 10pm on weekdays – and sure, many smaller stores and even the big shopping mall still opt to only open until 8pm, but thee are always one or two big grocery stores that are actually open until 10pm.
The quantitative difference between no open stores and one or two open stores may be small, the qualitative difference, however, is big.
If you're a mom and pop or speciality shop you may close between 7 and 9 (usually closer to 9), but most of the regional or national "stores" close between 10pm and 12am.
Grocery stores are even better, generally either closing at 12am or not closing at all. Wal-Mart, for instance, is mostly 24 hours, and Wal-Mart is very widespread.
It really depends where you live. In Maine, all grocery stores that I know of close no later than 9pm. I really think that is better too, especially since a lot of the employees are high school kids.
I think there is a difference. In German States, some states stipulate business hours†. In the US (and most other places) it's up to the businesses to decide their business hours.
Regarding the "supposed" part. I don't know exactly but I don't think there was some coordination or agreement between nations on who would do what.
Nearly everything closes at 8pm, even the grocery stores. In Berlin there were only two or three normal grocery stores I knew of that were open on Sunday (individual stores). Of course, restaurants and cafes are open a little later.
