Paul,
You do have a total of 40%. The two 10% awards are secondary to the PTSD, and if you read the 38 Code of Federal Regulations (38 CFR) you will find that you are being paid for 40% according to the Regulation. Strange calculations.
http://www.access.gpo.gov/cgi-bin/cfrassemble.cgi?title=200538
Check Part 4, Ratings for disabilities.