Lookup Tables are represented by lists of Column and Row input values under the Details tab and their corresponding output values under the Values tab. When deleting Row and Column values under the Details tab, the Row or Column output values at the end of the table are ultimately removed, and, the Column and Row Headers of the table are subsequently shifted/renamed.

Please see this article for more information on how to configure Table Lookup Elements in the Designer: Table Lookup Elements 

What Happens when you Delete a Row or Column in a Lookup Table? 

As an example, here is a Lookup Table: 

If you were to delete the Column named '1' from the Details tab, the last Column of the table will be removed and the Column Header names will shift one Column to the left

The original Column '1' has been renamed as '2' but retains its original values. Additionally, the last Column has been removed and has renamed the Column to its left (originally Column '2'), but contains its original values. 

It is best practice to ensure that you create the right amount of Columns and Rows so that you can rename and edit them, rather than delete them.