SharePoint adds the number because the name still exists because of the site collection not being completely removed from the content database. If you would try to create a third, it would be called ABC-3 and so on.
Changing the title, the site collection will still be named ABC-2 internally.
When you remove a site collection from the UI, the site collection does still exist but is marked for deletion for 30 days until it is deleted permanently.
The only way to remove the deleted site collection is to through PowerShell.
This step can be avoided by deleting the site collection itself using PowerShell from the first time.
You can type Get-SPDeletedSite to see what site collections that are marked for deletion.
Then you can use Remove-SPDeletedSite to remove the site from the database and thus be able to create a new site collection with the same name.
If you wanna delete all the site collection at the same time you can pipe the 2 cmdlts together:
Get-SPDeletedSite | Remove-SPDeletedSite
If you only wanna remove a single site, type
Remove-SPDeletedSite -Identity "http://farm/sites/sitecollToRemove"