I couldn't tell you why, however my instincts are telling me this might be a fake, and one of the best. One thing that I was critical of was the tehen kanamono; it does not look to be very detailed, at least to the standard that I have seen in 62 ken kabuto.
I agree that the tehen kanamono looks somewhat ill-fitting from the image shown, but I think the hachi is fine (although perhaps not of the highest quality and condition). It looks to be typical Haruta work. If anything, there may be some questions about the mabizashi (tsunamoto and shiwa), but it could be that someone just tried to clean it with an abrasive.