First, summon a llama with a strength of 1:

/summon minecraft:llama ~ ~ ~ {Strength:1,Tame:1,ChestedHorse:1}

Open its inventory, and put something in the second slot.

Then, change its strength to 2:

/entitydata @e[r=10,type=llama] {Strength:2}

The item will have visually moved! This doesn't seem like the correct behavior.

This happens because of the way the slots are numbered within the llama inventory: