A student could have the same weighted and unweighted GPA if that student has not taken any weighted courses. (Typically, weighted courses are AP classes and honors at some schools). All schools differ on how they weigh courses.
When you say your “grade is an 87”, that sounds like a grade in one class, whereas GPA is an average of your grades in all classes, all years of high school.
If you are talking about IB, which I know uses a numerical scale, disregard all the above as I know nothing about IB and GPA, hopefully others will respond.