SmartScan says 25GB trash, but Cleanup only finds 9GB?!

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This is quite confusing and frustrating!
I bought a new Lenovo Thinkpad L570 with Windows 10 Pro 64-bit. When it had finished updating Windows etc., I installed Avast Free Antivirus. I did the SmartScan, which said that if I bought Cleanup, it would delete 25GB windows trash. I already own Cleanup Premium, so I cancelled the buying procedure and found a way to key in the activation code for my Avast Cleanup. I started it and did a scan, but now it only found 9GB trash! What happened to the other 16GB trash??
On top of that: According to the Windows partition manager, Cleanup has only deleted about 3-4GB...?
What is wrong here?

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This is NOT satisfactory.
I ran the built-in Windows utility "Diskoprydning" (in Danish - I suppose it would be 'diskcleaning' in English) and it found 17.8GB old windows update files. It deleted them, and now there is 15GB more space on the disk. There should of course be 17.8GB new space, but 15GB is OK...
What is NOT OK, is that I have paid for the Avast Cleanup software, but not only does it not do the job, it also misleads me by saying that there is 9GB trash, when actually there is around 25GB!

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According to Microsoft, you only need their Widows Defender to give all the security needed.    So, adding third party software can be a waste of money and frustration.  Some of their disk utilities can do disk cleanup.  It's my suspicion that third parties incorporate Microsoft software into their utilities and make things easier for us dummies.   

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According to Microsoft, you only need their Widows Defender to give all the security needed.    So, adding third party software can be a waste of money and frustration.  Some of their disk utilities can do disk cleanup.  It's my suspicion that third parties incorporate Microsoft software into their utilities and make things easier for us dummies.

According to independent sources you need antivirus software, and not just Windows Defender. Avast Antivirus is free.
On windows 7 I also run COMODO Free Firewall 10.