Might it be the case that du follows links into other file-systems and includes the size of their contents?
-x, --one-file-system skip directories on different file systems
~ $ du --help BusyBox v1.10.2 (Debian 3:1.10.2.legal-1osso26+0m5) multi-call binary Usage: du [-aHLdclsxhmk] [FILE]...