The Unit ID (Unit #) and many other fields in the program allow for values which include both letters and numbers. The sort order of these fields is called "alphanumeric" which means that the order is determined from left to right, character by character.
As a result, an unexpected sort may occur such as the example shown here:
In order to force a numeric sort, add leading zeros for as many digits are in the numeric portion of the value. You can see how this affects the results in the example shown here: