Why do some of my participants get sent a receipt when I update event details via a profile form
When you update the status of multiple participants using a profile, any participant whose status is changed from a non-counted status to a counted status or vice versa will be sent an email.

Go to Administer > CiviEvent > Particpant Statuses to check which statuses are counted and which aren't. (In this case "counted" means "included in count of total registrations for the event".)