You're the second person to raise this issue of the ages of the various Targaryens from Daeron I through Daeron II. Some keen-eyed fan also pointed it out on the message board (here if you're interested in checking it out).
There's no good answer, except that I goofed. I worked out the ages (roughly) forward from Aegon II through the "present" of A GAME OF THRONES, but never thought to work them out backwards as well. It does indeed seem that they don't add up right, and that I am going to have to fiddle with the Targaryen succession. How, I don't know, but I'll make some change.