SGC's scale is different. A standard 7 is 84, so 83 is weak but not bad enough for 6.5. Edit* I read 83 as a weak 7.
84
NRMT 7
70/30 or better centering, slight wear on some corners, minor scratching, some print spots or speckling, and print lines or refractor lines are acceptable. Card may exhibit a slightly skewed (diamond) cut.
http://www.sgccard.com/grading_scale.htm