thanks to all to give your valuable suggestions
i think when i m calling analogReference(EXTERNAL) function before analogRead() then they would not config with other. i mean to say may be when we call analogRead() function then REFS1 & REFS0 must be reset that's why it's give us maximum values.
apart this if we didn't call ADC library and just make myself then it's work, so i think may be any other way to work with this library or have some bug in that.