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.

$$\ce{OHCH2CH2OH + PI3 -> [CH2I-ICH2] -> CH2=CH2 + I2}$$