I get the same.
There is never the exact amount as listed. Probably some resources it needs.
Same with external HD's. If its listed as 1TB, you actually get like 980GB.
Off topic, but this is actually for a different reason...
Hardware manufacturers list capacity in GB as defined in powers of 10. I.e. 1GB = 10^9 bytes = 1,000,000,000 bytes
Most software, however, defines a GB the correct way, in binary powers of 2; i.e. 1GB = 2^30 = 1,073,741,824 bytes
To simplify, here is how the units of storage are measured in binary:
1 KB = 1024 bytes (2^10)
1 MB = 1024 KB = 1048576 bytes (2^20)
1 GB = 1024 MB = 1048576 KB = 1073741824 bytes (2^30)
Using decimal:
1 KB = 1000 bytes
1 MB = 1000 KB
1 GB = 1000 MB
This is why a 500 GB Hard drive will only display as having ~465 GB when installed...just a difference of the definition of what a GB actually is. When most agree that using the binary definition is more correct, obviously hardware manufacturers prefer to use the decimal definition because it makes the capacity appear larger.