Prof gave me a B instead of a B+ for getting an 88%

I ended my grad school course with an 88.92% but when I checked my transcript I noticed that my professor had given me a B. Our graduate program allows B+ grades so I'm not sure why I didn't receive one when that is typically in the range. If the professor does not specify a grading scale in his syllabus, it would be reasonable to assume that an 88% would be a B+ right? Is it worth elevating to the Dean of the school if he refuses to change it? This grade is really important for my academic standing this semester.