vspeed problem

Hello,Strange,if I use the vspeed with auto-throttle to descent the descent folow-up the speed like Its using the 'lvl chg' option? I mean: when I like to descent 2100ft per minute it won't go alway's on 2100ft per minute but less, because its looking to the speed what he wants to maintain. So what ever I like to descent it alway's goes the speed wich I set in the auto throttle....like the level change option???If I tell the plane to descent 2100ft per minute it must go down 2100ft a minute and not less or more?What do I wrong?IVAO/KLM556