Probably because the cameras that do it this way have exposure bias steps that are set at 1/6 stop. By stating it as 12/6, it is easy to see the photographer moved the EC dial 12 clicks. If the camera displayed it as 6/3, then we would know the user had 1/3 stop steps selected when the photo was taken.
– Michael CJan 15 '15 at 00:59
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@mattdm, Exposure Bias tag 0x9204 in EXIF standard is specified as signed rational, that is represented by 2 numbers forming a fraction. Nikon are using '6' as denominator. 1/12 is internal calibration by the way.
– Iliah BorgJan 15 '15 at 01:44