Pension differences by region

I've noticed that average pensions in different regions differ by almost twofold — for example, in Kyiv it's over 9 thousand UAH, while in Ternopil region it's just over 5 thousand. Why is there such a significant difference between regions if all pensioners paid contributions to the same Pension Fund? Does this not violate the principle of equality, and can this be challenged if my pension is significantly lower than the national average?