That's because some schools don't do +/-'s. Therefore for these school a 4.0 or A would be 90-100, and notice there is no A-, B+, B- etc... for this category. Also some school have higher cutoffs, and that's why for some an A is at 93 and some at 94, so that's why you see a category where A is 94-100 and A- is 90-93, and another category where A is 93-100 and A- is 90-92. It all depends on your school system.
My schools doesn't do letter categories at all.
Well that's weird. In this case I wouldn't know exactly how that would be converted. Is it a US school? If yes, I would probably go by the +/- conversion where A starts at 93 and B+ at 87 just because that's pretty common, but of course I could be wrong.
Well I am in high school.
Loll. Well don't worry about how to calculate your GPA then. Just focus on your SAT or ACT, getting through high school and getting in a good College program because I guarantee you none of that will matter once you in College. Medical school will only require you to submit your College grades.
