When importing grades into the gradebook that do not have a corresponding gradable item already existing in the gradebook, Moodle will create the gradable item indicated in the spreadsheet with a max grade of 100 by default and add the scores. Since the Moodle 2.8+ gradebook does not allow you to alter the max score once grades have been applied, see MDL-48634, instructors need a way to indicate a max score within the spreadsheet or within the upload settings that can override the default max grade for the new item. Possibly the setting could be placed in with the mapping.
We are telling faculty to create the gradable items ahead of the import so they can have the correct max score in place, but this is not intuitive and counterproductive in their eyes.