The JPEG standard has no knowledge of color spaces. It simply compresses color components.
If is the specific file format (e.g. JFIF, EXIF, ADOBE) that specifies the color format. In most cases it is YCbCR. In some cases it is not (some adobe).
This link may explained the confusion
http://en.wikipedia.org/wiki/Yuv#Confusion_with_Y.27CbCr
YUV and YCbCR are similar, but different. If there is no color conversion, I have to believe that they have confused YUV and YCbCr.