Custom tipped arrows have their times wrong

When I give myself custom tipped arrows for instance this one:

/give @a tipped_arrow 64 0 {CustomPotionEffects:[{Id:20,Amplifier:0,Duration:320}]}

The item description says "Wither (0:02)" which is wrong because 320 ticks is actually 16 seconds instead of 2.

When I fire the arrow at something the effect takes 16 seconds to wear off, meaning that only the display is wrong