Ohh interesting, I didn't know that about Btrfs.
But these are FFS partitions (it's an OpenBSD machine). So I made some research and found this:
https://forums.freebsd.org/threads/understanding-negative-storage-space.28325/
Apparently FFS/UFS partitions reserve a percentage of the storage, so 101% of capacity means that some of that reserved space got written.