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Update query is meant to be faster than the regular field changing and saving pattern, thus it does not call the save() method, which handles updating auto_now fields, sending signals and so on. If you’re not sure what you’re doing, then it’s always a good idea to explicitly call the save() on a model. Advanced and “less restricted” methods such as update or bulk_create are faster and meant for editing data on DB level. From Django docs:
Finally, realize that update() does an update at the SQL level and,
thus, does not call any save() methods on your models, nor does it
emit the pre_save or post_save signals (which are a consequence of
calling Model.save()).
If you were hoping for a more technical explanation, then the update query probably doesn’t bother to check if the table has an auto_now field. It would require some data gathering and make the process slower. If you do want to update the field, you can update it explicitly.