Checkboxes are always checked regardless of supplied default_values.

When creating a form with type tableselect and you're using multiple values and in addition, you're assigning default values to your checkboxes to determine the checked state, the check states are always checked.

Example: You have a form that a user checks off some options and you save it to your database, then when retrieving the data, you'd get back the options the user saved. Therefore, you'll want to display those checkboxes as already checked.
However, when supplying the default values array, all the checkboxes are checked off regardless of your input from the array.