: When one of the alkyl groups is tertiary, the alkyl halide is formed from the tertiary alkyl group.

This is due to the reason that the reaction occurs by mechanism and the formation of products is controlled by the stability of the carbocation resulting from the cleavage of bond in protonated ether (oxonium ion). Since a tert-butyl carbocation is more stable than methyl carbocation, therefore, cleavage of bond gives methyl alcohol and the more stable tert-butyl carbocation. This carbocation then reacts with 1- ion to form tert-butyl iodide.