Why is there such variation in PhD length internationally?
In the UK system (or similar systems, like Australia) students enter their Ph.D. program with a stronger background due to earlier specialization, and they typically leave their Ph.D. program at a less advanced level than a Ph.D. from a comparable American school.  The difference is not in how fast people learn, but rather differences in what a bachelor's degree means and what a Ph.D. degree means.

Note that on specialization for undergraduates the US is the outlier, but in terms of Ph.D. outcome it's the UK that is unusual.  For example, Chris Parks writes, "Across Europe the view prevails that the three-year UK doctorate is too short and thus of inadequate quality compared with the more common four-year doctorate."