I think you're looking at it wrong.
The first Strike in the video, a new number never pops up. It just counts up. That's the way I've always looked at it. It's counting up and the last number displayed (the 488 or whatever) is the total damage done. In the second Strike, it the first hit is displayed, the second hit makes that number increase, and the third hit crits or something and displays a number. The total damage done is the second hit number and the third number together. I don't think it's a bug, it's just displayed weird.