Between which numbers? Satellite and Storagenode? Or Storagenode and disk usage? Did you exclude the trash usage?
GC might be failed last time… Did you see any error in the log? Or metrics about processed pieces?
1TB sounds fishy (unless you store 1PB 
on one node) Would be happy to help to debugging more, just send me more information (here/private or marton@ and storj domain)