Heehee... that's the observation I had too.
My kerises flex, but not as much as the ones that they use in the Barong dance. The metal of the kerises that they use seem to be very 'springy'. Also, the blades do not have prominent ada-ada like these 3 examples. I also notice that the kerises used in the dance are shorter Javanese/Madurese type. Not really Balinese kerises.