How to reactions of glycerol differ with excess HCl, HBr and HI ?

Both HCl and HBr give same pattern reaction while HI give different product 

Reaction of glycerol with limited amount of warmed HI gives allyl iodide.

While in the presence of excess amount of HI, then allyl iodide reduced to propene, which further reacts with HI and forms isopropyle iodide.

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