Just eyeballing the data:
A Bell Curve
The lowest rates (Tunisia and below) to a significant extent reflect poor governments (i.e. those with few economic resources per capita) with small budgets that can't afford to hire more police. The next lowest rates are associated with places that are affluent and low in crime (e.g. Finland). Indeed, one can imagine police staffing rates on a Bell Curve which is lowest in the most affluent/peaceful countries, and in the least affluent countries, and highest in the middle.
Added by Azor Ahai, called out some countries discussed on this page. n=121 countries I could quickly locate HDI for in that table
Economies of Scale And Urbanization
There are some economies of scale in policing, so very small population, small geographic area nations tend to have more police per capita. Many of the developed country high end outliers are such countries.
Not unrelated is that higher levels of urbanization, on average, are associated with more police per capita.
Other Outliers Mostly Due To Classification Issues
Palestine is an outlier and probably represents, to some extent, police forces being overstaffed in lieu of a military or paramilitary force which it is not permitted to maintain by Israel.
The low rate for China is also probably, in part, a classification issue with less of a clear divide between authority figures who constitute law enforcement and authority figures who do not. Likewise, China makes less of a clear division substantively between crimes and non-criminal anti-social conduct, treating these two things as more of a continuum.
I suspect that there are a few other outliers lurking which are driven by classification issues that defy the attempt at apples to apples comparisons that the list is trying to make.