I have a form field on a node create form that is an entity reference, referencing a view that displays all of the appropriate results. The view displays users with a relationship to a content field on the user, so the fields returned are all content. There is also an argument that takes the logged-in user ID. On the node create form, the field in question, when set to show unlimited results, only shows the last result from the view. I feel like I've gone over any technical reason this could happen, now I'm wondering if this is a bug with user views, or elsewhere. I haven't found anyone reporting this yet in my searches, so I thought I'd post it here.
I was having this issue as well, and it came down to the view returning a different entity_id than the one(s) I thought it should be returning. This happens because of incorrectly formed relationships.
A way to check this is to add a "dpm($options);" (if you're using the devel module) in the entityreference_options_list function of entityreference/entityreference.module file (just after the $options are returned from entityreference_get_selection_handler) to see what options it is retrieving from the view.
I started a new Entity Reference view that just returned the entities that I wanted, and then I built my relationships backwards from there to finally get the correct number of entities in my form.
I hope this helps!