A disc with a 25GB capacity is the equivalent of 23.28 gibibytes, depending on whether you’re using a base-2 system (binary) or a base-10 system (decimal). Also, some of that space is reserved as overhead for the file system, as well as error-correction information. So really, you’re getting a true recordable capacity of 23.28 gibibytes instead of the 25GB stated on the package for a single layer disc.
It is the same for dual layer discs. Normally rated at 50GB, in practice they can record about 46.57GB of data. Again, this is due to the differences in the way capacity is measured and the small percentage of space reserved for file system and error correction information.