A flickable needs to see 3 mouse move events before it starts moving the content.
This might be fair, because moving should not be detected accidently.
But when movement has been detected, the previous two mouse movements should be considered as well.
Example for a vertical movement of 12pixel, then 15pixel, then 20pixel:
On detection of the start of the movement, only the "20 pixel" movement is considered. Instead the first two moves of 12pixel and 15pixel should also be taken into account.
This would lead to an overall move of 47pixel (12+15+20) instead of 20.
With the todays implementation you see an "offset", which can be seen in a way that after ending the move your finger is not on top of the part of the flickable where it was when you started moving.