This is a popular form of protection on some systems, since it was difficult to write back to disk.
Sector sizes go up in multiples of bytes, for example: 128, 256, 512, 1024, 2048, 4096, 8192. However you can only fit around 6 KBytes in a sector, writing 8 KBytes would require more space than is available on a Double-Density disk and thus would end up wrapping around and overwriting the start of the sector under normal circumstances. To prevent this you would need to interrupt the writing before this overlap occurs, the track is marked as containing 8k of data which is physically impossible, in fact it contains somewhere around 6k (plus). The result is what we call a “6K Sector”.