Why does 項 signify both 'item' and 'neck'? I hope it's obvious why, but I don't think of &quot;necks&quot; as items. I can't imagine &quot;neck&quot; as as part of a &quot;list, collection or set.&quot; Why? Because I'm not trying to separate people's or animals' necks, and most humans don't deal with necks as &quot;an individual article or unit.&quot; Items are

An individual article or unit, especially one that is part of a list, collection, or set.

1.1.  A piece of news or information
1.2. An entry in an account.

Oxford Chinese Dictionary (2010) p 819.
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.