FAT-X (eh...FAT31,5 or somethin) uses a clustersize of 16384 bytes (16kb). This means that any file larger than 16k will span two-clusters. each cluster can only be used by one file. thus you get slack. I've got ~2gb slack on a 120gb :/
diagram:
1) Empty clusters
cluster1: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
cluster2: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
cluster3: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
2) Along came a 20,5kb file
cluster1: 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
cluster2: 11 11 11 11 10 00 00 00 00 00 00 00 00 00 00 00
cluster3: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
3) woha! a 6kb file!
cluster1: 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
cluster2: 11 11 11 11 10 00 00 00 00 00 00 00 00 00 00 00
cluster3: 11 11 11 11 11 11 00 00 00 00 00 00 00 00 00 00
geddit?