"Success has always been a great liar."
So you're playing Donkey Kong one day, one thing leads to another, and suddenly you find yourself entering your initials next to a score of over 500,000 points. Your very best, up to then, was 310,000. Holy crap, right? This was it! You've broken through your plateau! You're there! Next stop: kill screen.
Not so fast.
Most of what happens in Donkey Kong, in terms of movement of obstacles and enemies, is controlled by an ever-oscillating random number generator. This RNG determines whether or not a particular barrel will roll down a ladder, whether or not you are able to "steer" it, whether Kong releases it normally or as a "wild" barrel, where and how the fireballs and firefoxes move, when and from where the cement pans emerge, and so on.
As any experienced Donkey Kong player well knows, the random nature of each of these events, and how they cluster together, results in a game that can vary greatly in difficulty from one screen to the next. And every now and then, you'll play a game characterized by a kind of sustained easiness, where everything seems to go your way: you get a "free pass" on the conveyor screen every time, the firefoxes always spawn on the bottom on the rivets screen, you never have to retreat on a bad spring, the barrels steer exactly where you want them to (and Kong never seems to drop one in your face).
It's mathematically inevitable that you will see games like this. The result, when you do, is that you end up with a huge score, one that exceeds your average by as much as several hundred thousand points.
(Conversely, you'll sometimes have games that are horrifically unfortunate.)
For this reason, it is important, when evaluating your own ability, to go by the average of your best ten scores, and not by the very best score you've ever gotten. If most of your ten best games scored in excess of 500,000, that's meaningful, and almost certainly means that you're getting there regularly, and may even be due for a kill screen. But playing one, or even two, 500,000-plus point games, when the rest of your top ten are in the low 300s, means that the big games were almost certainly flukes, and that in reality you're still at the 300K level.
Unless you've had a major revelation or discovered a technique that makes some regular part of the game much easier, or you introduce a major deviation in your typical play strategy, there is simply no such thing as a single "breakout" Donkey Kong game. A breakout happens gradually, over the course of days or weeks and spanning several games. Even those periods where you find yourself playing "above the rim" for several games in a row can dissipate.
Be wary and don't fool yourself. The tendency will be to discount the rough, low-scoring games as temporary bad luck (they most likely are), but to think that one or two big games means that you've "cracked it open" (you most likely have not). Only your average score, not the one you miraculously hit that one day, can tell you how good you really are.