It's a mystery. I got my bike with 170 miles on the clock and from day 1 the speedo needle was all over the place at anything above an indicated 50 mph. At about 6,500 miles the cable broke at the front wheel end. I fitted a more expensive cable available in the UK and things were much better, but after a further 150 miles or so, the speedo stopped working completely. If I disconnected the cable from the speedo and turned the front wheel, the cable turned. If I reconnected the speedo end and disconnected the wheel end and put an electric drill onto it, the speedo worked. Put it all back together - nothing!
I recently fitted a cheap original Indian-made cable and so far, so good.
Tom