That would be because it is an ext3 (I think) partition, ext2/3/4 by default reserve 5% of the space for superuser use, so the superuser has space to work with to fix any problems caused by the partition being filled up, it also gives some space for the driver to work with to minimise fragmentation (though you typically want more like 10-15% for that), fragmentation doesn't cause as much of a performance impact on a SSD as it does on a hard drive, but you probably don't want it getting too bad. You can reduce or remove the reserved space using the tune2fs utility if you want to, but it is probably best to leave it as it is