organic chemistry - Reaction of ethylene glycol with PI₃ (P + I₂)

We know that $\ce{2P + 3I2 -> 2PI3}$

Ethylene glycol reacts with $\ce{PCl3}$ to give ethylene dichloride, but why does it gives ethylene when it reacts with $\ce{P + I2}$?

I don't really know why it happens so as it is quite contradictory because in case of primary alcohols, corresponding iodoalkane is formed.