Well all numbers have a cap for the purposes of making their math easier. Though 20 million does seem a little low for a value that continues to grow. Especially when you consider a 32-bit integer can represent over 2 billion and a 64-bit long can represent over 9 quintillion.
Looks like it's a cap they put in place long ago and just never bothered updating to account for bosses that simply have exponentially more health than they did back in the good old days.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.