If a list of items are assigned to a top-level item in another list, the items are not counted when the SUM function is used.
For example, there is a module with a list of events and a line item called "Assignment", which is a list format connecting to a list named "People". Some events are assigned to "All People", which is the top-level of the "People" list. If another module is created to show the number of events (through a function like Module1.Count[Sum:Module1.Assignment]) and names of the events assigned to each person, the events that are assigned to "All People" do not show up in the number of events.
Just to clarify, when you say "items are assigned to a top-level item in another list", do you mean that most of the list items are assigned to a parent, but these specific items are NOT assigned a parent (and therefore only appear under the list's Top Level item)?
If so, I can provide more clarification on how Anaplan treats un-parented list items in modules which may explain your results. Otherwise we may need some more details of the structure of your list and the configuration of the items within that list, preferably through screenshots if possible.
That makes sense, thanks for sharing those screenshots. Anaplan doesn't really have the sense of a "multi-select" drop-down, or the ability to select multiple items from a single list-formatted line item. So while you might expect selecting "All People" to result in each individual person in the list to be added to the project, Anaplan really considers that to be a separate list item. If you were to run a formula for Projects with All People assigned, then you would see your additional count, but in this scenario Users 1 through 3 are not included in the "All People" list item.
A few ways to solve this. You can write a formula where Module 2 adds +1 to the user if "All People" are selected. Or, more commonly when we see this need to add multiple list items, your Module 1 should actually have checkboxes as line items and Users as a separate dimension, where you would check the box for any User included in the project. Then a Line Item would say IF TRUE THEN 1 ELSE 0, which you would SUM in Module 2, to give you your correct values.
Happy to provide more details if you'd like, hope that helps!