Not that it would have any impact, it should be stateMappings="Closed=100,Open=0:99". Otherwise, the rest looks good.
As far as your observation, the binding controlling your blind is the culprit. As you can see, the skill sent command 70 and the state, controlled by your binding, only decreased to 75. Therefore, on the subsequent request, it used that state to determine the command value of 65.
Anyway, a new metadata syntax for the skill will be released shortly, including new door/window covering capabilities. This implementation will switch to using the UP/DOWN/STOP commands, opposed to specific percentage, on semantic actions for Rollershutter items.