When using "/achievement take" and then statistics (not acheivements) it takes some time to make it disapear from the statistics screen although it says it did it successfully (I tried it with Zombie killing) for example:
"/acheivement take stat.killEntity.Zombie" it says it did it successfully but it still showing in the statistics screen for some more time until it removes the statistics from there...

Leaving the world and getting back is fixing it and if I'm using the command some more it sometimes fixing it...