I was thinking that you get an additional life for every 10,000 points. Is this not what is supposed to happen? I only get an extra life at the 10K mark, not at the 20, 30, and 40K marks successively. Is there something wrong, or am I only supposed to get one life?

nope, just one at 10k, thats it

They probably did that because the scores would be too high if didn't. One person has the maximum score already, but many more would be able to achieve that if got a free life every 10,000 points. Considering you can start at any level you want, it becames extremely easy to get 10,000 points simply by obtaining 2 keys in the later levels, there would be hundreds of people with 999,999 points by now. It would ruin the leaderboard.

Pac-Man game is really a matter of skills. You don't really need a lot of life to get a high score. Soon that you've reached the hardest level, you'll end up consuming all of you life anyway. So it's the best way to improve your skill to acquire a high score.