Staves dealing less damage on average with full power up

The log on the left is damage dealt with one click. The log on the right is damage dealt with a full attack bar power up for each attack.

If I attack a brown spider with a single click, the mean value for damage dealt is 5.071428571
If I attack a brown spider with a fully powered up attack, the mean value for damage dealt is 4.5

Is this because the crit is 1? If so, That is quite the nerf. I am not expecting damage like buying arrows will get you with bows, but at least give me a bonus for winding up.