The more you evaporate the higher the OG as water is removed but not sugar.
What did you brew? Partial boil brewers need to use top off water to reach 5gal. When this is done conditions often lead to wort stratification (the water and wort weren’t mixed thoroughly) resulting in an incorrect gravity reading.
If you’re using extract and you use all of said extract and hit correct volumes of wort, the OG will be as stated in the recipe.