Nonstop backup takes less space after two days than single full backup?

I started using Nonstop Backup two days ago and now it takes just 18GB of space. This is much less than I expected, so I tried to create full backup* and it turned out to be 21GB! It is very odd that nonstop backup - which includes full backup plus hundreds of increments is smaller than single full backup.

If Acronis developed better compression algorithm, then why do they use it just for nonstop backups and not for regular backups?

Or does nonstop backup lack some features or have some drawbacks?

*maximum compression and folders that are excluded from nonstop backup excluded as well