In design view, the multi-select check boxes are displaying as expected, but during preview or after publishing, only a few of the check boxes are visible. Regardless of the settings, the same results appear.
So I notice you're pulling the bottom left 4 items, and the bottom right 3 items. That being said, there is a line there where the form has said "I only want to display these" - so that's a start into troubleshooting.
Any chance this is a repeating section? And, if so, are you sure that you've dragged all of your checkboxes into the grey area?
I am curious as to how you are getting two columns of checkboxes when your settings indicate that you have set this to one. Are these settings that you have snipped the same as the form? Is there any filtering applied to the List Lookup? Do you have the "Resize at Runtime" flag enabled in the advanced setting for the control?