It is very common that you have to move the sliders a considerable amount from the top resp. bottom position before reacting. I guess this is to compensate for mechanical tolerances to make sure the real slider values will be in the total range 0-127. My other (non-Yamaha) keyboards show this behaviour, too.
BUT - I agree fully that this dead range is wayyyy to big on the Genos. When I adjust a slider with a value range of 0-127, I have to move the slider approximately 6 mm from the bottom resp top position to get from 0 to 1 resp. 127 to 126. This is ridiculous. The total slider length is 45 mm, and within 12 mm there is no reaction, that's a dead range of roughly 25 percent. Bummer! This is just unacceptable for a high-priced TOTL keyboard.