This is standard behavior. When a filesystem is created, some of the available space is held in reserve, and is available only to the superuser. This provides a margin of error when you accidently fill the disk, but more importantly, it helps keep disk fragmentation to a minimum. I'm not sure about the default for FreeBSD, but the default for OpenBSD is 5%, so you could see up to 105% in df(1) output on that OS.
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