Something weird happened. I finished a not particularly good game at about 700k, looked at the score, blinked, looked again and yep, I had 1.7 million. There was no way I could have made that kind of score up to that level, so it had to be a glitch. Some games later, same thing.
Looked up strategywiki & gamefaqs, nothing.
Watched a vid of some guy playing not the same version of the game, but noticed that at one level he had 600k, at the next it was 1.6 mil. Turns out that the game has certain levels in which you have to place a balloon at a certain spot to get a 1 million points bonus. I managed to hit that spot accidentally in two games without even noticing the bonus message.
In the arcade version, the first spot is in the level after the 'aries get' interlude. Remove all balloons under the target, shoot a balloon into it's middle, add 1 mil to your score.
2,146,001.