That looks correct. Drive manufacturers list capacity in powers of 10 (1 KB = 1,000 bytes, 1MB = 1,000,000 bytes, 1 gigabyte = 1,000,000,000 bytes), whereas Windows uses powers of 2 (1KB = 1024 bytes, 1MB = 1,048,576 bytes, 1GB = 1,073,741,824 bytes).
The 256GB 840 Pro has a capacity of around 256,000,000,000 bytes. 256,000,000,000 divided by 1,000,000,000 is 256 manufacturer gigabytes; 256,000,000,000 divided by 1,073,741,824 is 238.4 gigabytes as reported by windows.
Everything checks out, the drive is fine.
Edit: Or did you mean there shouldn't be 138GB used by the things that have been installed so far?