That's a good question.
With China, the economic hardship argument would support a higher savings rate. They've had tough times historically, so they could be more hardwired to have a safety net.
I wonder if with Europe it's differences in their living structure? For example, as MMMers, we know how much your savings rate will increase if you just get a cheap used reliable automobile. In Europe, a large percentage of people don't even have cars, and almost no one has a big ass suv or truck. That could be a big factor. And healthcare/college costs are things that in the US are rising so fast in cost that it's likely contributing to a lot of the low savings rate. In Europe much of that is stabilized by socialism, so that could be another factor. There could just be a lot fewer shocks to the system in Europe that drain peoples savings and emergency stashes than there are here in the US.
I don't really know though, just throwing out guesses. It's a really interesting question.