Sounds like the navy is covering up some of the submarine losses.
According to the list I found here -
http://www.valoratsea.com/losses1.htm
Amberjack and Grampus were #10 and #11 of the 52 submarines lost in the war. So, allowing for the uncertainty inherent with submarine warfare the 7 mentioned in the article plus S-26 is not too far off. I wouldn't call coverup, all things considered.