You can't let a knockdown totally cancel out the other 2:50 of boxing irrespective of when occurred in them.
yes it is the reason it is stated just goes back to the years when there were different methods of scoring such as rounds, and 5 point must.
I have seen some one dominating get a flash kd and get up in a hurry, and dominate the rest of the round as a draw...I can't remember exactly when, but I have seen two of those rounds.