How is fragmentation calculated?

Hi,

I just downloaded Defraggler and analyzed a (non ssd) hard drive. According to defraggler there is 31% fragmentation.

The Windows 10 build-in defragmentation tool states the same drive with 0% fragmentation.

Why is there such a gap between those two tools?

Thank you for clarification.

Casper

Defraggler sees locked microsoft files which windows ignores their effect on the fragmentation. At least that's how I understand it

Thank you for your very fast reply :)

fyi: It's my second hard drive. Only data (holiday pictures/videos), no operating system. I don't think there are locked microsoft files.