Why does 項 signify both 'item' and 'neck'?
项 is originally means nape (back part of the neck), to distinguish from 颈 (front part of the neck). Later, 颈 became to mean neck in general.
Because 颈 is primarily for neck only, 项 later got borrowed into its other meanings.
Your neck holds up your head and supports it, thus the definition is then focused upon the head that is being held up by the neck.
Multiple heads became multiple individuals, and from there, the meaning evolved to become a classifier of items.