I have noticed this exact thing and have questioned several rental companies about it. There seems to be no obvious answer to why the residents of different countries should be charged different rates for the exact same rental with the same insurances and add-ons. Some answers have tried to explain this away as different (or "more appropriate") insurance cover or add-ons, yet closer examination show it is the exact same insurance cover and add-ons. The only consistent thing would seem to be in the case of US residents who (I have been told several times) are often able to provide their own insurance cover for a rental car, and therefore can get a lower overall rental cost if they exclude all the rental agency insurances.
In one discussion I had with a rental company some years ago, I asked what would happen if I lied about my residency to get a lower rate. The answer I received was that I could do that if I wished! Make of that what you will.
While I was living in the Netherlands, I used this to my advantage when travelling home to the UK and needing a rental car for the weekend. Since the question was always about where you were resident, and not your nationality or the issuing authority of your driving licence, I picked the residency that gave me the cheapest rental quote, reasoning that if I was ever questioned about it I could supply either a UK or NL address as appropriate. Often, despite making the booking as a NL resident, I then supplied my UK driving license and my UK address when collecting the car. It never even raised an mention at the rental agency desk.
Note however, I never actually tried to claim I was resident in a country where I didn't have an address. It suspect it might therefore get increasingly difficult if you were, for example, an American with a US drivers license, picking up a rental car in Europe while claiming to be a Chinese resident.