Why some of the 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 a notification: e.g 10 contacts updated from 20 selected.

This happens for the following scenarios:

Bulk updates → Status

• If selected contacts have already a Status that is selected in bulk: the contact's status will not be updated. E.g Jane Doe has a Status Lead and she is selected in bulk to be updated to a Lead then Jane Doe's Status will not be updated.

Bulk updates → Tags

• If selected contacts have already a Tag that is selected in bulk: the contact's status will not be updated. E.g Jane Doe has a Tag VIP and she is selected in bulk to be updated to a VIP then Jane Doe's Tag will not be updated.

Bulk updates → Owner

• If selected contacts have already the same Owner as the newly assigned one, no update will be applied to this contact after the change Owner bulk update is performed.

Bulk updates → Actions

• If Set Next Action is selected → Next Action text and date are entered → if Don't apply the new one is select in If a Next Action already exists then contacts with existing Next Action will not be updated.
• If Set Next Action is selected → Next Action date is chosen: contacts with no Next Action or closed sales cycle will not be updated.
• If Mark Next Action Complete is selected: contacts with no existing Next Action will not be updated.
• If Mark Next Action Complete and Close sales cycle are selected: contacts with existing Next Action will have their outstanding action marked complete but the sales cycle of contacts will not be closed if they have any Queued Actions assigned to you.
• If Delete Queued Actions is selected: contacts without a Queued Action will not be updated.
• Update a Next Action's date: only contacts with whom you have a Next Action will be updated. Contacts with Next Action assigned to other team members will remain untouched.

Bulk updates → More → Advanced updates... → Lead source

• If selected contacts have already the same lead source as the newly assigned one, no update will be applied to this contact after the change to the Lead source bulk update is performed. E.g Jane Doe has a lead source Referral and she is selected in bulk to be updated to a Referral → Jane Doe's lead source will not be updated.
   

Bulk updates → More → Advanced updates... → Contact address

• If selected contacts already have the new address and If content already exists: Don't apply is selected, contact address will not be updated.

Bulk updates → More → Advanced updates... → Organization address

• If selected contacts' organization already has the new address and If content already exists: Don't apply is selected, organization address will not be updated.

Bulk updates → More → Advanced updates... → Sync Status

• If selected contact's status is already in sync with others in their organization, no update will be applied to this contact's status when bulk update is performed.
• Selected contacts may be a part of a single-contact organization and therefore not relevant for a status sync.

Bulk updates → More → Advanced updates... → Sync Tags

• If selected contact's tags are already in sync with others in their organization, no update will be applied to this contact's tags when bulk update is performed.
• Selected contacts may be a part of a single-contact organization and therefore not relevant for a tag sync.

Bulk updates → More → Advanced updates... → Contact / Organization-wide custom fields

• If selected contacts already have the new custom field and If content already exists: Don't apply is selected, custom field will not be updated.