Ubercart module, understanding drupal_get_form
Its not related to drupal_get_form, its completely separated events sequentially fired in Drupal. 

step 1 - First in menu: argument %something loaded via something_load and passed to page callback as parameters.

step 2 - Second in menu: page arguments will passed to function specified by page callback.

step 3- Third in drupal_get_form: get first argument uc_attribute_form as form ID and use it for build form and get second argument 4 from menu which loaded in step 1. 4 means fifth argument in path 0-admin/1-store/2-products/3-attributes/4-%uc_attribute/5-edit

EDIT:
read more about hook_menu


  Registered paths may also contain special "auto-loader" wildcard
  components in the form of '%mymodule_abc', where the '%' part means
  that this path component is a wildcard, and the 'mymodule_abc' part
  defines the prefix for a load function, which here would be named
  mymodule_abc_load().