Left/Right Appear to move by quarter of span size?

[Dartec]

Hi, I have noticed that when I use left/right to move 'window' accordingly it appears to move by one quarter of the span size and not half as mentioned in the manual.  Thats if I am reading it correctly e.g. if the start is say 144.000 with span at 50.000 I would expect right press would change start to 169.000 but it actually shows 156.250, am I following correctly?  Regards Ray