Yes, there are invisible buffers, but the discrepancy doesn't come from this. Drive makers count the space in 1000 -multiple units (i.e. 1GB = 1000*1000*1000 bytes) whereas in computing units are usually count on 1024 -multiple (i.e. 1GiB = 1024*1024*1024 bytes)

So 1TB = 1000*1000*1000*1000 bytes/1024/1024/1024 = 931GiB (or 0.9TiB)