The differences in price are due to different insurance regulations in each country. For example some residents have lower quotes as most credit cards carry insurance and so they do not need to be charged extra.
Each different country has a different rate that needs to be charged according to the insurance terms and conditions of each rental supplier. These laws make some rates slightly higher than some others. Unfortunately this is unavoidable and part of the conditions of renting a car.
Also suppliers may be participating in marketing to specific countries to encourage tourists, by offering cheap rates.
The suppliers can ask for proof of residency to be eligible for any discounts - you must choose "I live in" - according to where you currently live. Then make sure you have a document to prove this.