Does that table already exist on another form with more columns?

From what I understand, the "add from variable" option is only meant to provide access to the variables already associated with that table or collection, just not displayed on the current form.

For example, if I had Form 1 and Form 2 in my business process and both use Table1. If I add a new column to the table on Form 1 it will not automatically be available on Form 2.

To add it to Form 2, I would use the "add from variable" option, which would provide the option of any variables connected to the table variable that are not already added for that particular form.