I think what they mean is that the 'sweet spot' exists because the maths just happens to work at those levels - the modifiers to the dice rolls are between 25% and 75% of the range of the dice roll itself (that is, between +5 and +15 on a d20), where at very low levels the modifiers are largely irrelevant (as they get swamped by the variance on the die roll), and at high levels the die roll is largely irrelevant (as it gets swamped by the modifiers).
When the designers put together 3.X, they didn't fully account for this oddity in the numbers, and so caused the 'sweet spot'. In 4e, the designers have noted this fact, and are building the game to suit. Hence, they have eliminated the 'accident of math'.
I might be completely wrong, of course.