Why some contacts selected in bulk are not updated

After using Bulk updates to modify multiple contacts, sometimes the number of contacts updated differs from the number of contacts selected, and you get the following notification:

OnePageCRM l yellow error message at the top displayed. X contacts updated from X selected. Why?

This happens in the following scenarios.

Bulk updates

Date - Reschedule feature

When changing the date of a Next Action:

Only contacts with an existing Next Action that is assigned to the user performing the update will be updated.
Contacts with Next Actions assigned to other team members will not be updated.

Actions

If Complete Actions is selected: Contacts without Next Action will not be updated.

If Close sales cycle is selected: Contacts with closed sales cycles will not be updated.

If Close sales cycle with Closing Comment is selected: Contacts with closed sales cycles will not be updated.

If Delete Actions is selected: Contacts without actions or with closed sales cycles will not be updated.

Status

If the selected Status label is already assigned to the selected contacts: The contact's Status label will not be updated.
E.g. Jane Doe has the Status label Lead already assigned and is selected in bulk to be updated to a Lead Status label, her Status label will not be updated.

Owner

If the selected Owner is already assigned to the selected contacts: The contact owner will not be updated.

Advanced bulk updates

Lead source

If the selected Lead source is already assigned to the selected contacts: The contact's Lead Source will not be updated.
E.g. Jane Doe has the Lead source Referral already assigned and is selected in bulk to be updated to a Referral Lead source, her Lead source will not updated.

Contact address

If you select If content already exists: Don't apply and if the selected contacts already have an address in the provided fields: The contact address will not be updated.

Organization address

If you select If content already exists: Don't apply and if the selected contacts already have an organization address in the provided fields: The organization address will not be updated.

Sync Status

The contact will not be updated and the sync will not be enabled:

If the sync for the Status label is already enabled for the organization.
If there is only 1 contact within the organization

Sync Tags

The contact will not be updated and the sync will not be enabled:

If the sync for the Tags is already enabled for the organization.
If there is only 1 contact within the organization

Contact Custom Fields and Organization Custom Fields

If you select If content already exists: Don't apply and if the selected contacts already have information stored in the Custom Field: The Custom Field will not be updated.