I'd expect that the default for a select style field would (A) be the default in the select ui list (B) be the default when it wasn't specified (like if the ticket was created via conduit or something). Instead if looks like only (B) is the case.

'addthis:scope' => array(
  'name' => 'Scope',
  'type' => 'select',
  'caption' => 'Like a T-Shirt',
  'default' => 'medium',
  'options' => array(
    'addthis:small' => 'small',
    'addthis:medium' => 'medium',
    'addthis:large' => 'large'
  ),
  'required' => true,
  'edit' => true,
  'view' => true,
  'search' => true,
  'copy' => true,
  'search' => true,
),

With creating a new ticket the selection box is set on 'small' to start with. When the ticket is created 'Scope from medium to small' showing that setting ''default' => 'medium'` is recognized . Worse the selection box "default" of value takes precedence over the default specified in the custom field.