It appears that gather quests are not 1 for 1 when you're using plugins that boost gather rates. For instance, Zlevels. 

It appears that if you have Zlevels set to 2x wood, quests may still think you gathered 500 when you gathered 1000. This may also carry over to other plugins that boost gather rates like SkinPerks, etc. 

This is very confusing to players.

My suggestion in your other thread should fix this issue too. The Gather bonus after the last hit doesn't get recognized in the newest version.