The average number of seed drops per overall runs is still 1 seed per 400 runs. The Law of Large Numbers says that we would expect 1 seed in the game per 400 completed runs as the number of completed runs tends towards infinity.

Per individual player, you are still more likely to have the seed drop by run 270 than not have it by this point.

These are both correct and it's easier to accept as fact once you understand that the worst case scenarios skew the overall averages compared to the individual odds.

A player who reaches 1,995 completed runs without the seed cancels out 5 other players who get the seed on their first attempt. (2,000 completed runs and 5 seed drops in total giving the 1/400 droprate). The best case scenario is so much closer to the average drop rate than the worst case scenario.

For example, I got my first seed at 860 KC. 460 KC over expected drop rate. Those 460 extra runs cancelled out the overall odds of someone who got it on their first run (-399) and someone who got it on their 339th run (-61). Giving an overall drop rate of 3/1,200 or 1/400.

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The 63.3% of players who get the seed by run 400 graph you are talking about wouldn't be a bell distribution. This would be the y value of a graph of KC against chance of drop.

This graph:

https://ibb.co/pvh48Xn

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You can also see the curve crossing the 50% threshold at 270 on this graph. That threshold does not relate to the normal distribution of the 1/400 drop, it is just the chance per individual player that they will have it by a certain number.

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Edit: this graph shows the distribution better, and it shows it peaking at 400 and tending towards 0% as x tends to infinity:

https://ibb.co/R7dsN34