User blog comment:Jack Pistol/Immortal Dovahkiin?/@comment-2040576-20121002202215/@comment-3492791-20121002203327

When a Dragon is killed by another Dragon, the victor takes the losers soul. Probably making the victor stronger. DB can do this, because they are like a Dragon. All dragons can, and DB is basically just another dragon. DB isn't special among dragons, only special among mortals.

"First born of Akatosh" just means Alduin was the first Dragon. Ever. Nothing really special beyond that. No one said his soul could not be absorbed. In fact it is believe that since you didn't he isn't dead.

Maybe Akatosh put him in timeout.