Why are System Administrators able to edit fields marked as Read-only on the page layout?
Under Administrative Permissions on the System Administrator profile, there is a setting called Edit Read Only Fields. If this is marked as True, any user that belongs to the System Administrator profile is able to edit Read Only fields. If the System Administrator profile is ever cloned, this setting carries over to the new profile.
Send us your feedback: We're always looking for advice to help improve our Knowledge Base! Please let us know if this article was helpful or provide feedback on how we can improve your experience here.