I would try to contact AMCAS about this one. (I know - easier said than done.)
At my undergrad school, the grading was A, AB, B, BC, etc. where an AB was a 3.5. Amcas treated it as a 3.5 as well when it did it's calculation. You shouldn't be penalized because your school uses the symbol "B+" instead of "AB" to mean a grade of "3.5". On the other hand, my senior year, they switched to an A-, B+ system where an A- was a 3.67 and a B+ a 3.33. AMCAS did convert these grades to 3.7 and 3.3.
Another question - when you select your school, does the AMCAS application automatically set-up the grade conversion? For 2 of the schools that I've been to, the grade conversion is not set up and I had to choose the conversion that is appropriate. Maybe your school is set up incorrectly. I would ask your pre-med advisor. There are probably other students at your school having similar difficulties.
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