How to get more numbers after comma for calculation

Projects Programming
1 
        double data1 = config[cstr][0];
        double data2 = config[cstr][1];
        Serial.println(data1, 6);    
        Serial.println(data2, 6);

        polyX[i] = data1;
        Serial.println(polyX[i]);

I am having the problem that the json string it prints the result as a json string : 9.908333
but when it needs to be calculated it is only: 9.9
This is GPS data, it's too much error
Hope to get help from you.

2

That prints six digits after the decimal point.

 Serial.println(polyX[i]);

That prints two digits after the decimal point, by default.

All calculations with variables declared double are carried out to full accuracy (16 to 17 decimal digits).

3

Kindly share the complete code
Also request you to please select appropriate category so that the experts can get into it properly